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THE PHYSIOLOGY OF ALIMENTATION. 
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(i « 


Ba a ed SESS SR OEE 5 che 


Frontispiece 
THoMAs GRAHAM 


AN INTRODUCTION TO 


THEORETICAL AND APPLIED 
COLLOID CHEMISTRY 


“THE WORLD OF NEGLECTED DIMENSIONS” 


BY 
DR. WOLFGANG OSTWALD 


Professor in the University of Leipzig 


AUTHORIZED TRANSLATION FROM THE EIGHTH 
GERMAN EDITION 


BY 
DR. MARTIN H. FISCHER 
Eichberg Professor of Physiology in the University of Cincinnati 


- SECOND AND ENLARGED AMERICAN EDITION 


NEW YORK 
JOHN WILEY & SONS, Inc. 
Lonpon: CHAPMAN & HALL, Limirep 
1922 


Copyricut, 1917, 1922, BY 
MARTIN H. FISCHER 


Stanbope Press 
TECHNICAL COMPOSITION COMPANY 
F. H. GILSON COMPANY 

BOSTON, U.S. A. ee 
oh # 


THE GETTY RESEARCH © 
_ INSTITUTE LIBRARY 


ia PB. 





TO 


Br. Martin GH. Hiacher 


Professor of Physiology in the University of Cincinnati 
IN SINCERE FRIENDSHIP 


i 





PREFACE TO THE SECOND AMERICAN AND 
EIGHTH GERMAN EDITIONS. 


. 


The editions preceding this have been exhausted in 
successively shorter periods — the last in a little more than 
six months. Such friendly reception touches my sense of 
responsibility. A thing so widely read needs to be tested 
and thought about. If the volume is to be more than a 
momentary cross-sectional view of colloid chemistry, if it 
is to continue in the eyes of a scientific world an introduction 
to the subject, repeated revision will be necessary. For 
this reason this edition has been gone over carefully, been 
improved and enlarged. I have previously emphasized 
the limits that must be set for enlargement and yet this 
volume contains some thirty additional pages. It seemed 
unnecessary to increase the number of demonstration ex- 
periments for I have collected many such in a laboratory 
manual.! 

Let me emphasize again that this volume is not a substi- 
tute for, but.only a stimulus to the seeing of colloid phen- 
omena for oneself. 


WOLFGANG OSTWALD. 
Lerezia, August, 1922. 


1 WoLFGANG OstwaLp, Kleines Praktikum der Kolloidchemie, Dresden, 
1920. 


Vil 





PREFACE TO THE FIRST EDITION. 


Tuts small volume is the literary result of a series of 
lectures which I gave during the winter of 1913 and 1914 in 
the United States and Canada upon the invitation of a 
number of American universities. Originally invited by 
five universities, I found the interest in the science with 
which this volume deals so great that their number grew to 
sixteen while the actual number of lectures demanded of me 
during some seventy-four days was fifty-six. Lack of 
time and strength compelled me then to forego the pleasure 
of accepting further invitations. By way of expressing my 
thanks and my appreciation of the friendliness and the high 
honor of these invitations and in order to send greetings 
once more to my many scientific friends on the other side, 
I beg to list the universities and institutions in which it was 
my privilege to discuss colloid chemistry. They are the 
University of Cincinnati (where I spoke under the auspices 
of the Cincinnati Society for Medical Research and the 
Cincinnati branch of the American Chemical Society); the 
University of Illinois; Columbia University, the College 
of Physicians and Surgeons and the College of the City of 
New York in New York City; Johns Hopkins University 
and the Johns Hopkins Medical School in Baltimore; the 
University of Chicago; the Ohio State University; McGill 
University; the Mellon Institute of the University of Pitts- 
burgh; the University of Nebraska; the University of 
Kansas; before the American Chemical Society in Indian- 
apolis; before the National Academy and the American 
Chemical Society in Washington. 

If I have omitted any institution or scientific body to 
which I had the pleasure of addressing myself and which 
in consequence did its share toward making possible the 
lectures given in this volume, I ask pardon. I admit that 
I had difficulty in remembering everything that happened 

ix | 


x PREFACE 


to me while living at what seems to be the customary 
American rate. I need to express my appreciation, also, 
of invitations received from the Massachusetts Institute 
of Technology, the University of California, Syracuse Uni- 
versity and a number of others — invitations which I regret 
it was impossible to accept. 

It hardly needs to be emphasized that upon such a tour 
the lecturer learns quite as much as his audience. The 
necessity of having his material so easily in hand that he 
may vary it according to the type and special wishes of his 
audience, or according to the time at his disposal, or to suit 
the viewpoint from which it is expected that his subject 
shall be handled — these things are of the greatest value to 
the lecturer himself. There is obviously much difference 
between the half-popular dissertation on colloid chemistry 
which is given twelve or thirteen hundred freshmen fore- 
gathered in a building ordinarily used for religious exercises 
and the talk which is given so select an audience as the 
American National Academy and the American Chemical 
Society meeting in the spacious halls of the Cosmos Club. 
And the theme of colloid chemistry is itself made to wear a 
different face, depending upon whether one talks the week 
through in Pittsburgh to workers interested chiefly in 
technical problems or whether one tries in two hours in the 
Johns Hopkins Medical School in Baltimore to discuss the 
relationships of colloid chemistry to biology and medicine. 

Besides such possibilities for arranging and rearranging 
his materials, other advantages accrue to the lecturer. He 
is in this way enabled to determine by experiment, as it 
were, what is the best form and the most easily intelligible 
one in which he can present his remarks, and what are the 
facts and thoughts which interest his audience most. He 
needs but to observe how it reacts to his mode of presenta- 
tion. He soon discovers what of that which he presents is 
not clear to his audience, is superfluous, or unduly long; 
what, on the other hand, interests them most; and in the 
discussions which follow a lecture he soon discovers whether 


PREFACE xi 


he has succeeded in making his main argument. clear. 
These things are possible, of course, only when the psycho- 
logical experiment can be made many times. How fruitful 
may be such an experiment tried in succession upon a series 
of new audiences is best evidenced, perhaps, by the fact that 
in the course of these lectures both the choice of material 
and its disposition in the various lectures underwent a steady 
change. It may fairly be said that what has been chosen 
for presentation in this volume is the product of this experi- 
ence. This, and the generous request of American friends 
that 1 print them, has led me to select the five lectures which 
I gave most often, to dictate them and to bring them out 
in this form. | 

There already exist a number of strictly scientific text- 
books treating of colloid chemistry and a number of more or 
less valuable introductions to colloid chemistry of a popular 
or semi-popular nature. So far as I know, however, none 
of these has tried to establish the right of modern colloid chemistry 
to existence as a separate and independent science while em- 
phasizing at the same time its great possibilities of scientific 
and technological application. The attempt to give a general 
survey of modern colloid chemistry as a pure and as an applied 
science and in a form readily intelligible to the general reader 
seems to be new. 

This volume makes its first appeal to such readers as have 
heard little or nothing of colloid chemistry. It was to 
several thousand of just such that I gave these lectures, and 
it was through frequent contact with them that I was led, 
time after time, to change my mode of presentation, and, 
I hope, to improve it. I had, however, another reason 
for thus addressing myself to such readers. There still 
exists, I think, too great an hiatus between the true signifi- 
cance, importance and application possibilities of modern 
colloid chemistry and the knowledge which the public has 
of this science. It is a fair statement that every scien- 
tifically cultured individual knows something about radio- 
chemistry. But that with radio-chemistry there was born 


xii PREFACE 


a twin science, the fruits of which are no less wonderful and 
the application possibilities of which to all possible branches 
of science, to technology and to industry are not only equal 
to but exceed those of radio-chemistry — this seems still 
largely unknown to the general public. I do not hesitate 
in consequence to designate this volume a propaganda sheet — 
for colloid chemistry. 

I am also presumptuous enough to believe that I shall, 
through this book, be able to render some of my colleagues 
in colloid chemistry a small service. It is a cause for re- 
joicing that the colloid chemist is being asked more and 
more frequently to address audiences upon the general 
fruits of modern colloid chemistry. These lectures may, 
perhaps, render him aid in such circumstances. I would 
especially emphasize the rather lengthy footnotes in which 
experiments are frequently discussed which have the great 
merit of always “‘going.”’ I have also written into this 
volume a number of not previously published opinions and 
experiments which the expert worker in colloid chemistry 
will readily discover for himself; and in the footnotes I 
have often tried to give expression to suggestions which 
come into one’s mind, one might almost say automatically, 
whenever one works long and hard in a given field. But 
the professional colloid chemist will, perhaps, be most 
interested in just what concepts and facts I chose for 
presentation, because they seemed to me to be especially 
characteristic of modern colloid chemistry. 

Because of the wealth of colloid-chemical papers and 
books, I could not hope to give references to more than a 
few. In choosing those which I did, I have selected for the 
most part such papers and larger texts as contain summaries 
of investigations and are ready guides to further literature. 

May this volume serve my readers as a guide into a long- 
existent but, until recently, scarcely recognized world of 
remarkable phenomena and wondrous mental concepts. 

WoLFGANG OSTWALD. 


GROSSBOTHEN, WALDHAUS 
July, 1914. 


TABLE OF CONTENTS. 


Po i SW ON ee 


Fundamental Properties of the Colloid State. Colloids as Ex- 
amples of Dispersed Systems. Methods of Preparing Colloid 
Solutions. 


dy UOT NDS se a a ee re 


Classification of the Colloids. The Physico-Chemical Properties 
of the Colloids and their Dependence upon the Degree of Dispersion. 


xill 


39 


85 


135 


193 





i 
FUNDAMENTAL PROPERTIES OF THE COLLOID 
STATE. 


COLLOIDS AS EXAMPLES OF DISPERSED SYSTEMS. 
METHODS OF PREPARING COLLOID SOLUTIONS. 





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COLLOID CHEMISTRY 


FIRST LECTURE. 


FUNDAMENTAL PROPERTIES OF THE, COLLOID 
STATE. COLLOIDS AS EXAMPLES OF DIB6- 
PERSED SYSTEMS. METHODS OF PRE- 
PARING COLLOID SOLUTIONS. 


I HAVE had the honor of being asked to tell you something 
of a new branch of physical chemistry, namely, colloid 
chemistry. Although I know full well that one should 
never begin a lecture with an apology, I feel that a few 
remarks are necessary before I enter upon my main theme. 

As you know, colloid chemistry is a relatively young 
science. Colloid chemistry was officially founded by the 
Englishman, THoMAs GRAHAM, some fifty years ago. 
Various papers, it is true, were written even earlier on sub- 
jects which we today regard as colloid-chemical. I need 
but to call to your mind the contributions of the German, 
BENS. JEREMIAS RICHTER, and of the Italian, F. SmuM1, 
which appeared in the beginning of the nineteenth century. 
Yet it is only in the last fifteen years that the facts of col- 
loid chemistry have become sufficiently large in number, 
that their relationships to each other have become suffi- 
ciently plain, and that laws regarding their behavior have 
been discovered which justify a discussion of colloid chem- 
istry as a separate branch of science. 

But, though still so young, the phenomena and the ideas 
incorporated under the term colloid chemistry are many. 
It is an almost universal complaint that colloid chemistry 
has already, —I beg you to note, already, — become so 
great that no one man can master it in its entirety. Espe- 

3 


4 COLLOID CHEMISTRY 


cially does the beginner find it difficult to get about in the 
confusing mass of colloid-chemical facts and theories. Nor 
has this rapid development in any sense ceased or even 
abated. The reverse is the case. New facts and new ideas 
are born daily as they were ten years ago, and a couple of 
colloid-chemical journals and some dozen text-books appear 
as the mere beginnings of attempts to organize and classify 
the available riches. No one knows this better than the 
authors and. founders of these text-books and journals. 

This is the point which I should like to emphasize before 
attacking my actual problem. It is impossible in my short 
series of lectures to give more than an outline of colloid 
chemistry. One can lecture for two semesters on this 
subject without doing it justice. It is far easier to give 
many lectures on colloid chemistry than only a few. What 
I bring you must represent a mere instantaneous photograph 
of what I regard as modern colloid chemistry. This fact 
may disappoint those specialists among you who would like 
detailed discussion of some of its special problems. I have, 
however, been told that what most of my audience desires 
is a survey of the field, so that what I say represents an 
effort to meet this wish. 


§1. 

The first question raised by any one approaching the field 
of colloid chemistry is this: what are colloids? About the 
same feeling is expressed in the questions: what are the 
most important characteristics of a colloid? Or, how can 
one determine quickly and simply whether or not a given 
substance is a colloid? This first lecture will attempt a clear 
and concise answer to these fundamental questions. To 
some of you it may appear that a whole lecture is too much to 
devote to these questions. I could, of course, be more brief 
were I simply to make two or three statements defining the 
concept colloid according to our present-day beliefs and 
were I then deductively to analyze and explain them. But 
such a deductive method of reasoning would, to me at least, 


FUNDAMENTAL PROPERTIES 5 


seem stupid. I believe that you will like it better if I try to 
. present the development of our concept in a more inductive 
and experimental manner. The answer to the question is 
by no means as simple and elementary as might at first 
appear. It has changed markedly with time and is today 
quite different from what it was in GRAHAM’s day or in the 
older text-books. How important it is to obtain a clear and 
short definition of the term colloid is clearly betrayed by 
the fact that there is often no end to debates in colloid 
chemistry because different authors have each different 
notions of what really constitutes a colloid. 

One might, at first sight, think it possible to decide whether 
a given substance is a colloid or not on the basis of its 
general chemical or physical properties. The word colloid 
is derived from the Greek xo\\a meaning glue. Thus one 
might think that all chemically complex substances are 
colloids. This conclusion is partly justified; it is not, how- 
ever, universally true. More important yet is the fact that 
while complexity of chemical constitution is likely to give rise 
to the colloid state, the converse does not follow. Among 
the many colloids I show you here,! you observe a whole 


series of very simple chemical composition (demonstration). 
1 T had at my disposal many dry colloids prepared by Paaw’s method 
with the aid of protective colloids. Simple solution of a few granules of 
these (when necessary with the application of heat) yields beautiful and 
lasting demonstration solutions. PaAau preparations of the various metals, 
metalloids, of mercury chromate and of manganese dioxid are easily pur- 
chasable. It is also easy to get colloid preparations of iron hydroxid (under 
the name of dialyzed iron oxid) and colloid carbon dispersed in an aqueous 
dispersion medium (india ink, aquadag) or in a mineral oil (AcHESoN’s oildag). 
One can also buy colloid dyes like congo red, benzopurpurin, night-blue and 
alkali-blue. It is an easy matter, also, to prepare colloid sulphids of the 
various metals by working with very dilute solutions containing a trace of 
gelatin. The same is true of colloid berlin blue, of silver iodid (KI + AgNOs) 
and of silicic acid (Na:SiO; + HCl). Examples of the so-called hydrated 
emulsoids (like gelatin) are also easily obtainable. These may be dissolved 
in cold water or when necessary, in hot. In the group with gelatin belong 
agar, starch, gum arabic, serum albumin, casein (dissolved in dilute alkali) 
and rubber. Collodion, viscose, etc., are also easily obtainable. The prep- 
aration of red.and blue colloid gold is discussed in the main text. Regarding 
further materials for demonstration see WoLFGANG OsTWALD, Kleines Prak- 
tikum der Kolloidchemie, Chapter IX, Dresden, 1920. 


6 COLLOID CHEMISTRY 


We have here colloid sulphids of the heavy metals, and here 
a whole series of colloid elements like gold, silver, sulphur 
and carbon. As of especial interest I show you colloid 
sodium chlorid, both as a milky liquid, and as a jelly of 
remarkably beautiful color.!. No one, of course, will at- 
tribute to sodium chlorid a complex chemical constitution, 
and yet you see it here in colloid form. Colloid water or 
colloid ice can also be prepared through the rapid chilling 
of toluene which contains water, or by pouring water into 
liquid air.2. There exists, therefore, no definite connection 
between chemical constitution and colloid state. In col- 
loid chemistry things are, therefore, different from those 
obtaining, for instance, in radiochemistry, where, as you - 
know, the observed phenomena are largely limited to 
elements of high atomic weight. 

It is also impossible to make a list from which one might 
then discover whether or not a given substance is a colloid; 
attempts at this were made years ago, but proved unsatis- 
factory from the start. Why is such the case? It is be- 
cause we already know too many colloids to make such 
cataloguing possible. While the preparation of a single new 
colloid was formerly regarded as of great interest, we are 

1 This milky colloid sodium chlorid was prepared by C. PAau’s method 
(see THE SvepDBERG, Herstellung kolloider Loésungen, 346, Dresden, 1909). 
A simpler and quicker method is that of L. Karczac, Biochem. Zeitschr. 
56, 117 (1913) which yields a particularly beautiful jelly-like colloid. Judg- 
ing by my own experience, it is best to use thionyl chlorid and sodium salic- 
ylate, which on double decomposition yield sodium chlorid and a complicated 
volatile thionyl ester. The dry sodium silicate is simply added to a few cubic 
centimeters of thionyl chlorid in a test tube, the sodium silicate being allowed 
to dissolve in the liquid through application of gentle heat. When about 
0.5 gram of the salt is added to 5 cc. of the liquid, there results, after cooling 
for an hour or two, a beautiful glass-like solid jelly exhibiting in striking 
fashion the so-called CurisTIANSEN refraction colors (for example, green 
by reflected light, red by transmitted light, etc.). These gels will keep in 
a closed tube for some weeks. If benzol, ligroin or some other substance - 
is used as a diluent, the colloid tends to go to pieces more quickly than if 
this is not done. 

2 Sez WoLtrGane OstwaLp, Handbook of Colloid Chemistry, trans. by 


Martin H. Fiscumr, second English edition, 106, Philadelphia, 1919, where 
references to the literature may be found. 


FUNDAMENTAL PROPERTIES < 


today familiar with methods by which we can at will convert 
‘whole classes of substances into colloids. There is little 
doubt that we have often worked with collaids and still do 
so without being aware of it. This is true, for example, of 
many of the dyes and of other organic substances of com- 
plicated chemical structure. Depending upon the kind of 
solvent used, depending upon the concentration chosen in 
any given solvent, one and the same substance may appear 
either as a “‘colloid”’ or a ‘‘non-colloid.’’ Tannic acid, for 
example, is colloid in water but not in alcohol; a simple 
crystallizing organic salt like tetraamyl-ammonium iodid 
is colloid in benzene, non-colloid in acetone even though it 
can be recrystallized from both solvents (P. WaLpEn); 
while a sulphonic acid studied by the Swedish investigator 
H. Sanpqvist behaved in dilute aqueous solution as a 
normal electrolyte but in higher concentration not only as a 
colloid but as a crystalline liquid.!~ Chemical constitu- 
tion, obviously, does not determine the colloid nature of a 
substance. As we proceed we shall encounter additional 
reasons indicating why such a listing of colloids is impossible. 

We might make attempts on other grounds to get an 
answer to our fundamental questions, but they would all 
prove unsatisfactory. From mere consideration of the 
chemical or physical properties of a substance, we simply 
cannot decide whether or not it is a colloid. TJ'o make such 
decision we need to study the properties exhibited by colloid 
substances under experimental conditions — we need to make 
a short, qualitative, colloid-chemical analysis. But when we 
do this we shall also get an answer to the question: how do 
we recognize a colloid? And by inductive and experimental 
methods we shall also get answers to the other questions: 
what are the important properties of a colloid, and, what 
are colloids anyway? 


§2. 
It is of interest that in making such a colloid-chemical 
analysis we use experiments which, in a certain sense, follow 


1 H. Sanvavist, Koll.-Zeitschr., 19, 113 (1916). 


8 COLLOID CHEMISTRY 


the historical development of the whole subject. The 
concept colloid was born of experiments on diffusion. With 
the fundamental phenomenon of diffusion all of you are 
familiar. If the lower half of a cylinder is filled with the 
solution of a colored salt and pure water is then carefully 
poured upon it so that admixture does not occur, the salt 
slowly wanders upwards into the pure water even when all 
vibration, etc., is shut out. 

THOMAS GRAHAM was among the first to experiment 
extensively in this field and to follow his results quanti- 
tatively. An important element in GRAHAM’s work lay in 
the fact that he studied many different kinds of substances. 
He noted great quantitative differences in their diffusibility. 
While some dissolved substances, such as salts, acids and 
bases, showed a considerable diffusion velocity, he noted 
little or no diffusion in the case of gelatin, albumin, silicic 
acid and aluminium hydroxid. Those which diffused but 
little or not at all Grauwam called colloids. This simple 
observation constitutes the foundation of the science of 
colloid chemistry. 

It is easily seen that such diffusion experiments are hard 
to carry out quantitatively when the pure solvent is simply 
laid over a solution, for slight variations in technic and 
currents due to temperature differences disturb their ac- 
curacy. It is well-nigh impossible to demonstrate such 
experiments on ‘‘free”’ diffusion to a large audience. It 
is, however, possible to carry out these experiments in a 
more stabile form, the basic principle of which was also 
recognized by GRAHAM. It rests upon the fact that the 
velocity of diffusion in dilute gels — but only in dilute 
ones — is practically the same as in the pure solvent. I 
show you here several test tubes half filled with three per 
cent gelatin (demonstration). Upon them were poured a 
series of colored solutions which were then permitted to 
diffuse down into the gelatin for some days. You observe 
that the blue copper sulphate and the yellow picric acid 
have penetrated deeply into the gelatin. On the other 


FUNDAMENTAL PROPERTIES 9 


hand, the tubes into which colloid gold, silver, iron hydroxid 
and congo red were poured show little diffusion. The series 
demonstrates clearly the different degrees of diffusibility 
of the different substances, just as Granam first observed. 
Let me emphasize at once that 
as shown in Fig. 1 these simple 
diffusion experiments prove of 
great service in any attempt to 
determine the colloid (a) or non- 
colloid (6) character of a given 
solution. 

There is another way of over- 
coming the difficulties incident to 
experiments on free diffusion 
which permits, perhaps, a still 
sharper distinction between dif- 
fusing and non-diffusing sub- 
stances. Presumably in an effort 
to overcome the experimental 
difficulties incident to his syste- 
matic study of many solutions 
GRAHAM resorted to the follow- 
ing: Suppose we imagine a mem- 
brane of some sort, capable of tak- 
ing up the solvent, to be intro- 
duced between a salt solution and 
its pure solvent. If a parchment 
paper tube, such as I show you 
here; is filled with a solution and ie. 1.—Diffusion of a col- 
the whole is then placed in a, loid (a) and 2 “true” solution 
beaker filled with the pure solvent ( ite solid gelatin. 
as shown in Fig. 2, it is clear that diffusion may occur through 
the parchment paper without being subject to disturbances 
due to vibration, etc., at the surface of contact between solu- 
tion and solvent. Diffusion experiments of this type were 
also first made by GraHaAm; in fact, he gave them a special 
name, calling the diffusion of dissolved substances through 





10 _ COLLOID CHEMISTRY 


membranes, dialysis. He found that the substances capable 
of free diffusion passed through these membranes, while 
those incapable of diffusion did not. A distinction of non- 
colloid from colloid solutions could, therefore, be made 
by dialytic means. Such a parchment 
membrane is, obviously, nothing but a 
thin colloid membrane. 

Parchment paper is, of course, not the 
only substance from which such mem- 
branes may be prepared. Pig bladder, 
fish bladder, sausage casings, reed tubes, 
or collodion may be used. They may 
be used in connection with and to 
cover special pieces of apparatus, such 
as bells or rings. I show you here a 
number of such dialyzers, directing your 
especial attention to two, constructed 
according to GRAHAM. ‘To prepare arti- 
' ficially a dialytic membrane, as one of 
collodion, filter-paper capsules need 
simply be soaked in a collodion solu- 





Fie. 2.— Dialyzer ar- 


ranged for colloid analy- : 
sis: tion after which the solvent for the col- 


lodion is allowed to evaporate.' 


! Following the method of G. Matrirano, collodion capsules are best 
prepared by dipping clean test tubes into liquid collodion. To get the col- 
lodion evenly distributed, the tubes are turned in the air while evapora- 
tion of the solvent is taking place. After being thus dried, the collodion 
films are stripped from the tubes. Dialysis thimbles may also be prepared 
by coating the inside of Erlenmeyer flasks with collodion and pouring off 
any excess. Everyone who has worked with these collodion sacks knows 
that their preparation is associated with a whole series of small technical 
tricks. The parchment thimbles of Schleicher and Schill are convenient for 
many colloid-chemical purposes but are rather costly. They often have 
an acid reaction and should therefore be washed in boiling water before use. 
The saturation of filter-paper thimbles with collodion, as described in the 
text, is much simpler and cheaper, since filter-paper thimbles can be obtained 
in all sizes and are relatively cheap. What is most important, however, is 
that diffusion capsules thus prepared are very strong. They may also be 
used as osmometers. ; 

Further improvements making for more rapid dialysis, employ the prin- 
ciple which I have described for the production of the so-called ‘‘spontaneous 


“FUNDAMENTAL PROPERTIES 11 


As you see, we have already uncovered experimentally 
two characteristics of colloids. I imagine now that I hear 
you say: that is all very simple; colloid solutions are 
solutions which do not diffuse and which do not dialyze. 
This would constitute an experimental definition of colloid 
solutions. No doubt we have discussed two of the most 
important experimental characteristics of colloid solutions, 
but when we look at the problem more closely we discover 
that these do not suffice to characterize them fully. More- 
over, a little thought reveals that this definition rests upon 
certain theoretical assumptions which may not be taken for 
granted. Let us take up this point for a moment. 


§3. 

Trouble arises from use of the word solution in our defini- 
tion. What do we mean by this term? Let us for the 
moment free our minds of all special hypotheses regarding 
its nature. What we regard as characteristic of a solution 
is that it represents a molecular distribution of one substance 
in a second. Is this requisite fulfilled in the case of colloid 
solutions? Are ‘‘molecules”’ floating about in them? The 
older authors, including GraHam, believed this to be true 
even though they did, of course, think that there was some 
sort of a difference between the molecules of a colloid and 
those of a non-colloid. A first attempt to define this con- 
sisted in pointing out a possible physical difference between 
the molecules of the two, as illustrated, for instance, in the 
phenomena of allotropism. Sulphur, for example, in its 
different allotropic forms possesses different physical char- 
acteristics. There is, for example, rhombic and hexagonal 
sulphur and sulphur as Sy and Sd. In some such ill-defined 
manner GRAHAM and his followers accounted for the differ- 
ences between the molecules of a colloid and a non-colloid 
solution. Carry Lma, one of the best known of American 
colloid chemists, gave his paper on the colloid solutions of 


ultrafilters’’ (Koll.-Zeitschr., 22, 72, 148 (1918); see also the footnote on 
page 48). 


12 COLLOID CHEMISTRY 


metals the title, ‘‘Allotropic Modifications of Silver” actu- 
ally meaning new colloid forms of it. 

Suppose, for the moment, that we assume this view to be 
correct and that we actually do deal both in solutions of 
colloids and of non-colloids with molecules of the orthodox 
type but possessed of different physical properties. There 
really do exist many similarities between colloid and ordi- 
nary molecular solutions. Thus many colloid solutions like 
those of red gold, of congo red or of berlin blue are just as 
clear to the naked eye as molecular solutions of fuchsin or 
copper sulphate (demonstration). But colloid solutions 
also behave like ordinary molecular solutions in that they 
pass unchanged through paper filters and even through most of 
the very fine porcelain or clay filters. I can prove this to you 
with any of the colloids here on the table, as with this colloid 
gold or colloid indigo (demonstration). These phenomena 
emphasize the great similarities and close relationships 
between colloid and ordinary molecular solutions. 

Let me show you an experiment which will recall your 
first days in qualitative analytical chemistry. I have here 
a saturated solution of mercuric cyanid to which I add some 
hydrogen sulphid. You see that mercuric sulphid is pro- 
duced (demonstration). A thick precipitate is formed 
which quickly settles and which we can then readily filter 
off (demonstration). Only a practically colorless solution 
passes through the filter. Let me now repeat the experi- 
ment, but this time I shall use a very dilute cyanid solution 
(demonstration). As you see, mercuric sulphid is again 
produced which must this time also be solid for it is insoluble 
in water or in a dilute solution of hydrocyanic acid. I again 
pour the dark brown solution upon a filter, but you observe, 
we encounter what is so unpleasant to the analyst: the 
precipitate goes through (demonstration). What are we 
to do? Is the ‘‘notoriously”’ insoluble sulphid of mercury 
prepared from the dilute solution also a colloid? We know 
that it is a precipitate, and a precipitate of a solid substance, 
for mercuric sulphid at room temperature and in the pres- 
ence of water can be nothing else. If the concentration is 


‘FUNDAMENTAL PROPERTIES £3 


merely raised, or the solution is left to itself for a time, or 
if we add salt, we obtain a solid precipitate from this brown 
liquid, as every analyst knows, yet this brown liquid which 
has passed through the filter and which contains the sulphid 
precipitate looks just as clear to the naked eye as any 
ordinary filtered molecular solution. We can also carry out 
diffusion and dialysis experiments with this finely-divided 
precipitate, for it will keep for days. 

In the series of tubes illustrating diffusion there is one 
filled with just such a mercuric sulphid precipitate as we are 
discussing and you notice that none of it has wandered down 
into the gelatin. ‘The precipitate therefore behaves like a 
colloid in this regard also. But may we under the circum- 
stances still continue to speak of molecular division? Are 
all the other colloids we have before us nothing more than 
such finely-divided precipitates — the idea seems rather . 
plausible — are they nothing but very fine suspensions of 
insoluble substances, nothing but ‘‘ mechanical’’ suspensions 
or emulsions produced by mixing an insoluble solid or liquid 
into a second liquid menstruum? No doubt these facts will 
convince you that inability to diffuse and to dialyze are 
alone not sufficient to characterize colloid solutions. Mere 
suspensions of finely-divided precipitates also do not diffuse 
or dialyze. 


$4. 


Connected with these peculiar relations of the colloids to 
the ordinary solutions and of the colloids to the mechanical 
suspensions there is a most interesting and vital debate. 
On the one hand, investigators have tried to make colloid 
solutions simply a subdivision of molecular solutions and 
something different from ‘‘mechanical”’ suspensions; on the 
other hand, another group has emphasized the similarities 
between mechanical suspensions and colloids and placed 
these two together over and against the molecular solutions. 
Their views may be indicated as follows: 

Mechanical suspensions Colloids Molecular solutions 
Ee SL a 


14 COLLOID CHEMISTRY 


They have all tried, in other words, to group the three 
types of substances under two headings. The discussion 
has at various times leaned now to one side, now to the other, 
as the one or other partisan believed he had at last dis- 
covered a conclusive difference between the two classes. 
Thus, suspensions of the coarser precipitates are rather 
turbid, while many colloids are clear to the naked eye. But 
even FaraApAy learned to use a special method of illumina- 
tion which permits the recognition of slight turbidities, and 
so was able to show that red colloid gold is also turbid. 
Those who grouped colloid solutions with the suspensions 
at once used this fact as evidence for the correctness of their 
view. On the other hand, those who grouped the colloids 
with the true solutions pointed out that in typical, filterable 
colloids one can no longer make out the individual particles 
under the microscope, and they used this in support of 
their view. It was a believer in this view, R. Zsiamonpy, 
who was able by optical methods to demonstrate the pres- 
ence of individual particles in typical colloids, and so again 
to prove the inadequacy of this classification. 

The discussion has not yet been settled. But most 
interesting is the fact that not a single colloid chemist any 
longer troubles about it. The discussion has simply dis- 
appeared. And why? Because modern colloid chemistry 
teaches that there are no sharp differences between mechanical 
suspensions, colloid solutions and molecular solutions. There 
is gradual and continuous transition from the first through the 
second to the third. It 1s best to regard all three from the same 
viewpoint and first to emphasize their similarities. After 
this has been done, their special peculiarities may be taken up. 

This constitutes, perhaps, the most important conclusion 
of our whole modern colloid chemistry. In this lecture I 
can only ask you to take my word for the truth of this 
continuity of the three classes. The next will be largely 
devoted to proving it to you. 


FUNDAMENTAL PROPERTIES 15 


85. 


What now is there common to suspensions, colloid solu- 
tions and molecular solutions? 

Briefly stated, the physical and chemical properties change 
in periodic fashion in all three. Let us imagine a suspension 





Fic. 3. — Diagram illustrating the concept, dispersed system. 
Density, Coefficient of Refraction, etc. 





ee LOLI ES (ATT fb HL) 


Particles 
Fig. 4.— Diagram illustrating the concept, dispersed system. 


of quartz particles in water. Were we to measure, by 
appropriate means, the changes in the coefficients of re- 
fraction in such a suspension and plot the results, we should 
obtain such figures as are shown in Figs. 3 and 4. We 


16 COLLOID CHEMISTRY 


should encounter a periodic increase and decrease in the 
coefficient of refraction depending upon whether we were 
striking a quartz particle or the suspending medium. The 
same periodic change, not only in the coefficients of refrac- 
tion but in all other physical and chemical properties, would 
be encountered, no matter in which direction we went 
through the quartz suspension. But such periodic changes 
in properties would also be encountered in any solution in 
which the substances are in a state of molecular division. 
In molecular solutions, too, must appear points where the 
physico-chemical properties of the solvent predominate, and 
again others in which the properties of the dissolved mole- 
cules, combined perhaps with the solvent, predominate. 
Thus in such an electrolyte as a dilute salt solution we know 
that the positive and negative electricities must follow each 
other in succession. Other physico-chemical properties, 
like density, must change similarly, but such periodic 
changes in a molecular solution must occur within smaller 
spaces (within what we call molecular distances) than in a 
quartz suspension. What has been said regarding suspen- 
sions and molecular solutions must hold for colloid solutions 
also. In all three the physical and chemical properties 
show periodic changes in space. 


86. 

This view is a central one in modern colloid chemistry. 
Colloid chemistry speaks of the disperse structure of these 
solutions, applying to them the general terms dispersed 
systems or dispersoids. A dispersed system is therefore 
nothing more than one in which the properties change 
periodically in space. 

To address myself for a moment to the physical chemists 
among you, it is clear that this definition is more inclusive 
than that represented by the terms ‘‘ polyphasic”’ or ‘‘hetero- 
geneous.”” When we speak of a polyphasic system, as 
represented, for instance, by a quartz suspension, we mean 
that periodically a whole serves of properties changes at 


FUNDAMENTAL PROPERTIES 17 


once. Practically all the physical and chemical properties 
change as we pass from the one phase into the other. But 
our concept of dispersion makes no assumptions whatsoever 
regarding either the kind or the number of the properties 
which are changed in space. If you will call to mind, for 
example, that RONTGEN rays are regarded as little more 
than oscillating systems of electrically-charged masses, the 
electrons, you will observe that it is possible to construct 
dispersed systems which consist, practically, of but one 
form of energy.! 

The term dispersed system is valid, therefore, not only 
for so-called “heterogeneous”’ systems but also for “‘homo- 
geneous”’ ones, as represented by molecular solutions. The 
term means less than heterogeneity, yet it contains more 
than the term homogeneity. ‘This discussion may sound 
too theoretical to please you, but I believe that you will soon 
recognize for yourselves how illuminating it is when applied 
to the problems which we are about to take up, and how 
fruitful are its practical applications. 


§7. 


Coarse suspensions, colloid solutions and molecular solu- 
tions are all to be regarded as dispersed systems and to be 
studied together under this common heading. In this 
fashion all the old quarrels arising out of the attempt at 
classification of the colloids on a dualistic basis are disposed 
of.2. But how do the three types of systems differ from each 


1 The concepts of quantity which have recently been applied to different 
kinds of energy and combinations of energy are adaptable, in large measure, 
to dispersed systems, provided it be,.remembered that, as ordinarily used, 
the term ‘‘quantity’’ embraces only units possessing a maximal degree of 
dispersion. 

2 It has been argued occasionally that this expansion of colloid chemistry 
to dispersoid chemistry is a matter of words only and that I have merely 
introduced the ‘‘useful terms” dispersed system, dispersion medium, degree 
of dispersion, etc. But the reader will perhaps see for himself (from the first 
and second chapters of this volume, for example) that the mere business of 
words is as nothing compared with the importance of the concepts which have 
been developed. 


18 COLLOID CHEMISTRY 


other? They differ, first of all, in the nwmber of the periodic 
changes encountered in the unit volume. As is readily 
apparent, the number of periods, or the degree of dispersion, 
increases while we pass from the coarse suspensions through 
the colloids to the molecular solutions, as shown in the 
following diagram.! 


DISPERSED SYSTEMS. 


a 
Coarse Suspensions . Colloids Molecular Solutions 








Direction of increasing degree of dispersion 


The degree of ‘‘subdivision”’ of physical and chemical 
properties is greatest in the molecular systems and least 
in the coarse suspensions. Molecular systems belong to the 
most highly dispersed, coarse suspensions to the least dis- 
persed systems. Colloid solutions occupy a middle posi- 
tion. There is, of course, not the slightest reason for 
assuming that any sudden change occurs in degree of dis- 
persion as we pass from the coarsely dispersed to the colloid 
systems, or from these to the molecular. Not only is there 
no theoretical reason against such a view, but there is no 
practical one either. As I shall show you in detail in the 
next lecture, we know dispersed systems of every degree of 
dispersion vn nature. 

It is well, perhaps, to give you some concrete este eae 
of this. I show you here a series of different kinds of sulphur 
(demonstration). In this first bottle I have the familiar 
large yellowish-green crystals; their structure is so coarse 
that we can hardly speak of them as dispersed systems. In 
this second bottle I show you sticks of sulphur; these have 
a crystalline structure but the crystals are already so highly 
dispersed that they are hardly visible to the naked eye. 
This third bottle contains flowers of sulphur which represent 
under-cooled droplets of sulphur that are but fractions of a 

1 The lecturer will obviously not write this diagram anew in every lecture 


but simply develop it from a single diagram mounted once and for all time 
before the audience. 


FUNDAMENTAL PROPERTIES 19 


millimeter in diameter; they show, in other words, a micro- 
scopic degree of dispersion. Here I show you colloid sulphur 
in a watery ‘‘dispersion medium”’; it is a milky liquid from 
which the sulphur separates out only very slowly; in a drop 
of it placed under the microscope you do not see any particles; 
this system is therefore still more highly dispersed than the 
preceding one. The fifth bottle contains another largely 
colloid sulphur, namely, sulphur dissolved in benzol; it is ‘a 
scarcely turbid, yellowish fluid in which the sulphur is still 
more highly dispersed than in the watery solution.t. And 
here, in this sixth bottle, I show you molecularly dispersed 
sulphur, in the form of the well-known solution of sulphur 
in carbon disulphid. 

You observe, therefore, how one and the same substance 
may appear in all possible degrees of dispersion. Other 
substances can, of course, also be made to assume different 
degrees of dispersion. For example, there are the large 
crystals of sodium chlorid, the more highly dispersed ones 
constituting common table salt, the colloid preparations of 
sodium chlorid that I have already shown you, and its 
ordinary molecular solution. 

But these facts also show you how through our concept of 
the dispersed system our main problem of the relation of 
colloid solutions to molecular solutions and to coarse sus- 
pensions finds a simple answer. We pass from the one into 
the other gradually and it is entirely arbitrary at which 
point we decide to insist on lines of division between the 
three classes. On theoretical grounds we cannot say what 
degree of dispersion is characteristic of any one of the 
classes. On practical grounds, however, we can settle upon 
values which are suitable as a basis for division. These 
coincide with degrees of dispersion at which certain methods 
used in the investigation of dispersed systems either fail or 
can first be used to advantage. 


1 See J. AMANN, Kolloid-Zeitschr., 8, 197 (1911). 


20 COLLOID CHEMISTRY 


88. 


A practical division between coarsely dispersed and colloid 
systems can be made, for instance, microscopically. It 
follows from the theory of microscopic vision that we cannot 
see individual particles of a diameter of less than half a wave 
length of light. By employing micro-photographic methods 
which enable us to work with the short waves of ultra-violet 
light, we obtain as the limit of microscopic vision a value of 
about one ten-thousandth-of a millimeter or 0.1 u. This 
value is used — let it be noted, altogether arbitrarily — as 
marking the transition from coarse to colloid dispersions. 
Other methods of investigation, such as filtration, yield 
similar values. The pores of the best grades of hard filter 
paper (No. 602 of Schleicher and Schill) are about 1 » in 
diameter; those of clay and porcelain filters about 0.2 to 
0.4 uw. These values therefore approximate those obtained 
by microscopic means. It is characteristic of typical col- 
loids that they pass through these filters while coarse sus- 
pensions do not. 

If now we seek a line for the division of colloids from 
molecularly dispersed solutions, we may begin by asking 
the physical chemists about the size of molecules. By 
methods which we cannot discuss here, they have decided 
that typical molecules have a diameter of one ten-millionth 
to one one-millionth of a millimeter, in other words 0.1 to 
1.0 pu. The diameter of a very large molecule like that of 
starch has been calculated as 5 yw». But, as you know, 
starch dissolved in water shows marked colloid properties, 
so this value comes within the realm of colloid dimensions. 
We are familiar with colloid-chemical methods to be dis- 
cussed later, like optical ones, for example, which also begin 
to fail us when we reach dimensions approximating one one- 
millionth of a millimeter. It has, therefore, been agreed, 
again arbitrarily, of course, to draw the line between colloid 
and molecular at this point. 

The region of dispersity within which the colloids lie is 


FUNDAMENTAL PROPERTIES JA 


therefore bounded by particles having, on the one hand, a 
diameter of one ten-thousandth of a millimeter, on the other 
one of one one-millionth of a millimeter, as indicated in the 
following diagram: 

DISPERSED SYSTEMS 


a... 
Coarse Dispersions Colloids Molecular Dispersoids. 
CO -——_“— 

Increase in degree of dispersion 
0.1 u to 1.0 py 


Periods greater than | Pass through paper filters; | Periods smaller than 


0.1; do not pass cannot be analyzed micro- 1.0 uu; pass through 
through paper fil- scopically; do not diffuse filter paper, cannot 
ters; microscopi- or dialyze. be analyzed micro- 
cally analyzable. scopically; diffuse 


and dialyze. 


Dispersed systems lying within these middle limits are 
called typical colloids, but let me again emphasize that we 
deal with purely arbitrary divisions and that we are familiar 
with transition systems of every degree of dispersity not 
only between coarse dispersions and colloids but between 
these and molecularly dispersed systems. 


89. 

We are now able to answer the question raised at the 
beginning of this lecture: what are colloids? According 
to modern colloid chemistry they belong, with mechanical sus- 
pensions and molecular solutions, to the group of the dispersed 
systems, differing from the suspensions and the molecular solu- 
tions only in the special value of their degree of dispersion. 
This is the theoretical definition of the colloids. From an 
experimental point of view — and under this heading we 
shall get the answer to our questions regarding the means 
by which we may recognize colloids — the colloids differ 
from the coarse dispersions in that they cannot be analyzed 
microscopically and in that they pass through ordinary filters. 
The colloids differ from molecularly dispersed systems vn that 
they do not diffuse and do not dialyze, which molecular solu- 
tions do. But should you ever find occasion to express or to 


22 COLLOID CHEMISTRY 


make use of this modern definition, do not forget to add that 
there exist transition systems not only between coarse disper- 
sions and colloids but between colloids and molecularly dis- 
persed solutions. The colloids merely represent a realm 
differentiated for practical purposes from a continuous serves 
of systems. 

If all this is true, some important corollaries follow. If 
colloids are ‘‘nothing but”’ systems of a certain sub-molec- 
ular degree of dispersion, it follows that every substance 
may appear in the colloid form or be made to appear so, for, 
theoretically at least, we know that for every substance 
there must exist a second substance in which the first is not 
spontaneously soluble in molecular form. You can see for 
yourselves how well experience bears out this conclusion. 
The table here is covered with a fairly large number of 
colloid preparations and I have told you that there are many 
hundred others. There are so many that it is impossible to 
list them all. These facts are the best sort of confirmation 
of the teaching that every substance may be obtained in 
colloid form, or expressed in the words of the Russian 
investigator, P. P. von Werrtmarn: The colloid state 1s a 
universally possible state of matter. 


$10. 


Our diagram of the dispersed systems also enables us to 
predict by what general methods a given substance may be 
brought into the colloid state. There are two such. We 
may begin with a non-dispersed or coarsely dispersed 
system and increase its degree of dispersion until colloid 
dimensions are reached, or we may start with a molec- 
ular system and allow the molecules to combine, aggre- 
gate or condense until the colloid state is reached. Their 
further growth is then interrupted. The former of these 
is known as the dispersive, the latter as the condensive 
method of producing colloids.! 


1 This general distinction between condensive and dispersive methods of 
preparing colloid solutions originated with THE SvEDBERG. 


‘FUNDAMENTAL PROPERTIES oO 


Many different methods, or, better expressed, many dif- 
ferent types of energy may be used either to disperse or 
to condense the molecules of any substance. Colloids may 
be prepared by employing not only mechanical energy but 
chemical or electrical energy, or even heat and light. Of the 
many possible methods I can show you but a few. I shall 
show you a chemical condensation method and an electrical 
dispersion method. 

A particularly interesting colloid is that of gold which I 
have already showed you as an intensely reddish-violet or 
bluish liquid. This colloid gold was prepared even in the 
days of the alchemists by the reduction of gold salts with all 
kinds of organic substances, such as urine. B. J. RICHTER, 
M. Farapay and many other investigators have since then 
studied it. In order to obtain it by a method of chemical 
condensation I begin with a molecularly or ionically dis- 
persed solution of gold chlorid to which sodium bicarbonate 
has been added until neutral to litmus. I need now to 
reduce the gold chlorid to metallic gold, but this must be 
done in such a way that the resulting gold remains so highly 
dispersed as not to exceed colloid dimensions. As you know, 
gold chlorid may be reduced by many different kinds of 
substances, especially organic ones. You need but dip your 
finger into the solution when it becomes stained a bluish- 
violet by the colloid gold produced through the reducing 
action of the organic substances contained in the skin. You 
are frequently told in colloid chemistry that the preparation 
of fairly stabile colloid gold is a delicate undertaking, for 
which not only the purest distilled water is necessary but 
accurate quantitative work as well. If these things are 
ignored, red gold is rarely obtained, but violet or blue gold 
appears instead. I want to show you a method by which 
- we can always obtain red colloid gold even when we work. 
but roughly. 

This Erlenmeyer flask contains about 100 cc. of ordinary 
distilled water. I add a few drops of a neutralized one per- 
cent solution of gold chlorid, and, after mixing, a few drops 


24 COLLOID CHEMISTRY 


of a very dilute solution (about 0.1 percent) of tannin. We 
need not work any more accurately than this. Only not 
too much gold chlorid or too much tannin must be used. 
The completed mixture should be practically colorless 
(demonstration). I now heat it over a Bunsen burner for 
one or two minutes, shaking it constantly. You observe 
that even before the mixture begins to boil it assumes a 
cherry red color. JI may now add more gold chlorid or 
more tannin as necessary and thus obtain an almost red- 
dish-black solution. The experiment will succeed even 
with ordinary tap water.! 

You may be interested in knowing how blue or violet gold 
is prepared. A method which furnishes blue gold, as cer- 
tainly as the previously described experiment yields red, 
consists in adding a few drops of a very dilute solution of 
hydrazin hydrochlorid to a dilute, neutral solution of gold 
chlorid. The blue color appears almost immediately if 
enough of the reducing material is added (demonstration). 
If I add but little, a violet color is obtained. If too con- 
centrated solutions are employed, the gold becomes bluish- 
black or greenish-black. It is then no longer colloid but 
precipitates out in microscopically visible particles.? 


1 This method for the production of stabile red colloid gold is interesting 
because it really ‘“‘works”’’ every time, provided only neutralized gold chlorid 
is used and the work is carried out in sufficiently great dilutions. If the red 
color does not appear immediately after heating, more tannin and more 
gold chlorid may be added alternately without endangering the possibilities 
of getting the desired red color. It must be cautioned that the hot solution 
may not at once be diluted with cold water. This is likely to bring about 
a change from the cherry red to violet. After the colloid solution has been 
cooled, dilution will not markedly affect the color. This assured method 
for producing red colloid gold (I have performed the experiment countless 
times with all kinds of materials and even when only tap water was at hand) 
seems not, as yet, to have been described in the literature. Its success seems 
to depend upon the fact that the tannin acts not only as a reducing sub- 
stance but, to a certain degree, also as a protective colloid. 

2 This method also always “works.” <A tiny crystal dissolved in some ~ 
20 cc. of water yields a solution which for most purposes acts as a sufficiently 
strong reducing mixture. For other simple methods see WoLFGaANG OsTWALD, 
Kleines Praktikum der Kolloidchemie, 2, Dresden, 1920, 


‘FUNDAMENTAL PROPERTIES 25 


These demonstrations illustrate chemical condensation 
methods. We begin with molecularly dispersed systems, 
free the gold molecules and then allow them to coalesce into 
larger aggregates. We choose the conditions for our experi- 
ments so that the aggregation does not proceed to the point 
of yielding coarsely dispersed precipitates but ceases as soon 
as the condensation has attained colloid dimensions. What 
are the experimental conditions which must be maintained 
in order to attain this end? 

You have seen for yourselves that I have worked only with 
dilute solutions. As I emphasized in the experiment on the 
precipitation of colloid mercuric sulphid, we obtain a colloid 
precipitate which will pass through the filter only if the 
precipitation is produced in very dilute solutions. Let me 
show you another example of this dependence of degree of 
dispersion of the precipitate obtained upon the concentra- 
tion of the reacting solutions. 

I have poured together in this first beaker two very dilute 
solutions of iron chlorid and potassium ferrocyanid. The 
resulting precipitate of berlin blue is so highly dispersed — 
it is a colloid — that the liquid is intensely blue yet appears 
perfectly clear to the naked eye (demonstration).! In this 
second beaker I have poured together two somewhat more 
concentrated solutions of the same materials. You observe 
that a bulky, dark-blue precipitate has formed, above which 
there remains the slightly colored dispersion medium. 
Evidently, therefore, the degree of dispersion is less in this 
second beaker, while the size of the individual particles of 
the precipitate has been increased, simply by working with 
more concentrated solutions of the reacting materials. 

I show you now two still more highly concentrated, 
practically saturated solutions of the two reagents. When 

1 For demonstration purposes it is best to use large glass cylinders or 
parallel-walled museum jars. These should be lighted from behind by 


means of an arc lamp, the light from which is made to pass through ground 
glass or paper, 


26 COLLOID CHEMISTRY 


I pour these together! and stir with a glass rod (demonstra- 
tion) you note a remarkable fact: the two liquids set to 
form a cheesy paste so stiff that I can turn the beaker upside 
down without losing its contents (demonstration). Please 
recall that the paste resulted from the mixture of two mobile 
liquids possessing in themselves no high degree of viscosity. 
I now make the following experiment: when I take some of 
this thick precipitate and 
stir it into a large volume of 
distilled water (demonstra- 
tion) I again obtain a clear 
blue liquid which is fairly 
stabile and which is also 
colloid, as I can prove to 
you by filtering it (demon- 
stration). It looks, there- 
fore, as if, by the use of Concentration of the reaction mixture—> 
very highly concentrated Fic. 5.— Influence of the concentra- 
reaction mixtures, the size tion of the reaction mixtures upon the 
of the precipitated particles size of particles of a precipitate. 

is again decreased. ‘This is really the case, as has been shown 
in detail by P. P. von Wermarn. The precipitate is 
coarsest when medium concentrations are employed. The 
size of its particles decreases both with decrease or increase 
in concentration of the reacting substances. A curve 
illustrating the relation of size of particles to concentration 
of the reacting solutions would, therefore, show a maximum 
in a region of medium concentration, as indicated in Fig. 5. 


Size of particles of precipitate ——— 


$11. 


Because of the importance of this von WEIMARN law in 
colloid synthesis by chemical condensation methods, I show 
you a number of microphotographs illustrating these facts 


1 Since the saturated potassium ferrocyanid solution contains much less 
salt than the iron chlorid solution, about 2 cc. of the chlorid solution must 
be added drop by drop to 10 cc. of the cyanid. The iron chlorid i is poured 
into the cyanid — not the other way about. 


“FUNDAMENTAL PROPERTIES 27 


(demonstration).! The pictures are photographs of barium 
sulphate precipitates, made by pouring together barium 
cyanid and manganese sulphate. They show the changes 
in the character of the precipitate in passing from mixtures 
of dilute solutions to those which are more concentrated. 
Fig. 6 presents the precipitate formed on mixing go> 
normal solutions. As you see, the picture shows nothing. 
This is just what it is intended to show. At this concentra- 
tion we obtain a colloid precipitate of barium sulphate, 
and, since colloid particles are not visible microscopically, 
the photograph could, of course, show nothing. Fig. 7, 
obtained with ;;455 normal solutions, begins to show par- 
ticles. The enlargement is about 1 : 1500. As we approach 
the higher concentrations of s+, normal to = normal 
(Figs. 8 and 9) we observe a gradual increase in the size 
of the particles. ‘The photographs are all on the same scale 
and may, therefore, be compared directly with each other. 
At still higher concentrations, 345 to 7; normal, actual 
crystals begin to appear, as evident in Figs. 10 and 11. 
In these concentrations the maximum size for the indi- 
vidual particles has been attained. From this point on, as 
we pass through the higher concentrations of 4, 4 and + 
normal, you observe that the size of the particles again 
decreases steadily (Figs. 12, 13 and 14). In still higher 
concentrations, such as 2 to $ normal (Figs. 15, 16 and 
17) we obtain the paste-like precipitates which I showed 
you in the case of berlin blue. The pictures of such precip- 
itates appear as solid films torn in various ways. It is 
still possible to make out that these films consist of minute 
crystals. In the most highly concentrated, almost satu- 
rated solutions the microphotograph again shows nothing 
(Fig. 17). 

It is amusing that in the classic German formulas for 


1 See P. P. von Wermarn, Kolloid-Zeitschr., 2 (1907, 1908); see also 
his Zur Lehre von den Zusténden der Materie, Dresden and Leipzig, 1914. 
Not all the photographs appearing in the original are reproduced herewith. 
For lecture purposes it is best to project diapositives. 


28 


COLLOID CHEMISTRY 





- FUNDAMENTAL PROPERTIES 





29 


30 


COLLOID CHEMISTRY 





Fia. 10. 





Fie. 11. 


FUNDAMENTAL PROPERTIES 





Hig. 712: 





ol 


COLLOID CHEMISTRY 


32 





Fic. 14. 





15 


G. 


Fi 


“FUNDAMENTAL PROPERTIES 





PHiGee ly 


33 


34 COLLOID CHEMISTRY 


making colloid solutions through chemical condensation, 
very dilute solutions are always insisted upon while the 
formulas of the American colloid chemist CarzEy LEA are 
equally insistent upon the use of concentrated ones. In 
keeping with the American way of doing things Carry 
Lea’s formulas begin by calling for several grams of gold 
chlorid. | 


§12. 


I show you next a dispersion method of producing colloid — 
solutions in which use is made of electrical energy. It is 











= 
() 








Fic. 18.— Apparatus for dispersing metals electrically. .. 


G. Brepia’s method of dispersing metals. I have here two 
silver wires fastened into a stand in such a way that the 
ends may be approximated by turning a micrometer screw 
(Fig. 18).1 A five- to eight-ampere current obtained by 
sending the ordinary 110-volt current through a rheostat 
is now sent through these wires. Their tips dip into dis- 
tilled water which has been slightly alkalinized with a trace 
of sodium bicarbonate. I turn on the current, and by 


1 This exceedingly useful arrangement was placed at my disposal by 
Professor J. STIEGLITZ in the University of Chicago. 


FUNDAMENTAL PROPERTIES 35 


regulating the micrometer screw, allow a tiny arc to form 
between the wire tips under the water (demonstration). 
You observe how dense dark brown or greenish clouds 
emanate from the tips of the wires and remain suspended in 
the water. 

This greenish-brown liquid is one of colloid silver some- 
what contaminated with colloid silver hydroxid. The solu- 
tion is perfectly clear to the naked eye and passes easily 
through filter paper. Other metals may be colloidally dis- 
persed in the same way; in fact by making use of special 
methods such as oscillating electrical discharges, low tem- 
peratures and organic dispersion media, THE SVEDBERG 
has prepared even the alkali metals in the form of beauti- 
fully colored colloid solutions. 

We can also prepare colloid solutions by exposing metal 
plates to ultraviolet light, by heating alloys and cooling 
them suddenly in water and by grinding powders for long 
periods of time. 


§13. 


The main conclusions then to which this lecture has led 
may be summarized as follows: 

Colloids are dispersed systems, in which the diameter of the 
dispersed particles in typical cases lies between one ten-thous- 
andth and one one-millionth of a millimeter. They are dis- 
tinguished experimentally from molecularly dispersed systems 
by the fact that they do not dialyze; and from coarse dispersions 
by the fact that they cannot be analyzed microscopically. Col- 
loids pass through filters readily, while coarse dispersions do 
not. Transition systems exist between colloids and molecular 

1 For a discussion of colloid synthesis through exposure to radiant energy 
see WOLFGANG OstwaLp, Grundriss d. Kolloidchemie, 1. Aufl., 302, Dresden, 
1909; Tue SvepserG, Kolloid-Zeitschr., 6, 129, 238 (1910); for the prepa- 
ration of vanadic acid by sudden cooling see E. Mt.imr, Kolloid-Zeitschr., 
8, 302 (1911); for the preparation of colloids by trituration see WOLFGANG 
OstwaLp, Grundriss der Kolloidchemie, 1. Aufl., 292, Dresden, 1909; see 
also C. Brenepicxs, Kolloidchem. Beih., 4, 260 (1913), who describes the 


production of colloid gold by trituration as practiced in the seventeenth 
century, as well as G. WecELin, Koll. Zeitschr., 14, 65 (1914). 


36 COLLOID CHEMISTRY 


solutions and between colloids and coarse dispersions. The 
colloid state represents a universally possible state of matter. 
There is no reason why every substance may not be produced in 
colloid form. It may be accomplished either through the dis- 
persion of non-dispersed or coarsely dispersed substances, or 
through the condensation of molecularly dispersed systems. 
To these ends not only chemical but mechanical, electrical and 
other kinds of energy may be used. 


| iM 
CLASSIFICATION OF THE COLLOIDS. 
THE PHYSICO-CHEMICAL PROPERTIES OF THE 


COLLOIDS AND THEIR DEPENDENCE UPON 
THE DEGREE OF DISPERSION. 





he 1.5 





SECOND LECTURE. 


CLASSIFICATION OF THE COLLOIDS. THE 
PHYSICO-CHEMICAL PROPERTIES OF THE 
COLLOIDS AND THEIR DEPENDENCE 
UPON THE DEGREE OF DISPERSION. 


THE previous lecture dealt with the fundamental facts 
and problems of colloid chemistry. I tried to show you 
how the concept of the colloid has assumed a new meaning 
by having been grouped with the dispersed systems. The 
colloids are dispersed systems distinguished by the special 
value of their degree of dispersion. This view emphasizes, 
in other words, that there are no sharp differences between 
coarse suspensions, colloids and molecular dispersoids. We 
pass gradually from one into the other, and their properties 
change as smoothly. It is the purpose of today’s lecture to 
prove the truth of this principle of continuity. 


$1. 

Let me first direct your attention to a further corollary to 
the conclusion that colloids represent dispersed systems in 
which the degree of dispersion has a special value. You 
have already seen how this modern definition compels the 
conclusion that every substance may appear in a colloid 
state, and how it systematizes also the general methods by 
which colloids may be prepared. The basis for a third 
conclusion may be introduced as follows. 

I have here a coarse suspension of infusorial earth in water 
(demonstration). As you know, infusorial earth consists of 
the silicic acid coverings of minute organisms. There is no 
doubt, of course, that this mixture is one of solid particles in 
water. The same is true of this black precipitate of gold 


made by adding more gold chlorid and more reducing agent 
39 


40 COLLOID CHEMISTRY 


to the blue colloid gold I showed you yesterday (demonstra- 
tion). The particles of gold in blue and red colloid gold 
must also be solid, for we cannot think of gold at ordinary 
temperatures as existing in any other form. Coarse dis- 
persions of solid particles in a liquid are known as suspen- 
sions; colloid dispersions of the one in the other, as suspension 
colloids or suspensoids. In the terminology of P. EHREN- 
BERG the latter are also known as granular colloids. 


§2. 

I show you in this flask two liquids which hardly mix with 
each other in molecular form, namely, water and benzol; I 
have added a little iodin to the latter to give it a violet color 
(demonstration). When I shake the flask you observe that 
I obtain mixtures of the one in the other, in other words, 
two emulsions, one of benzol in water and another of water 
in benzol. We have here divided two liquids into each 
other. As you know, this subdivision of two liquids into 
each other can be carried very much further, as seen in the 
milk of animals and plants, in cod liver oil emulsions, ete.. 
The mixture may be so highly dispersed that high power 
microscopes are necessary to distinguish the separate drop- 
lets. This is true, for example, of human milk and of the 
milk of some rubber plants. Do we know emulsions of 
a colloid degree of dispersion? ‘There are, of course, no 
reasons against the existence of such colloid emulsions or 
emulsoids, for we know that liquids dissolve in liquids and 
the principle of continuity underlying our classification of. 
the dispersed systems clearly indicates that colloid emul- 
sions must exist between the extremes of coarse dispersions 
and of molecular dispersions of one liquid in another. I 
show you here two types of such colloid emulsions or, to use 
the technical terms, of such emulsion colloids, emulsoids or 
droplet colloids. As an example of the first, [ show you 
colloid sulphur (demonstration). As even the older in- 
vestigators knew, droplets of liquid under-cooled sulphur are 
formed which slowly solidify or crystallize whenever sul- 
phur is precipitated in watery solution. We have many 


_CLASSIFICATION OF THE COLLOIDS 41 


reasons for believing that the microscopic and even colloid 
particles found in such mixtures retain this liquid form.! 

In illustration of this second type of liquid-liquid colloids 
I could show you many examples, in fact, these are probably 
the best known and most widely distributed of all the col- 
loids. Glue, gelatin, agar, protein, starch, rubber and 
collodion belong to this group. We shall discuss it in detail 
in the next lecture, when we shall also take up the differences 
existent, for example, between an emulsoid of sulphur and 
one of gelatin. 


§3. 


I have in this third flask an example of another coarsely 
dispersed system, a coarse dispersion of a gas in a liquid 
(demonstration). You see before you a saponin foam. 
There is no reason why a gas cannot assume a dispersed form. 
Are there dispersions of gases in liquid in which the degree 
of dispersion attains colloid dimensions? No doubt there 
are, for we are familiar both with coarse dispersions and 
with molecular dispersions of gases in liquids, but in illustra- 
tion of them we can cite but few examples.” They are seen 
in the opalescent critical mixtures observed when liquids 
are evaporated, or gases are being liquefied in regions of 
critical temperature and pressure. 


§4. 


Certain objections may be raised to this classification of 
the colloids based on the state of aggregation of their con- 
stituents. The term itself, state of aggregation, refers to 
material in mass. It evidently loses its meaning as we 
approximate the molecular dispersoids in our progress 
through the series of dispersed systems. We cannot speak 
of the state of aggregation of a molecule. But how about 
colloid particles? May these still exhibit different states 
of aggregation? Our diagram of the dispersed systems and 


1 See the monograph of 8. Opén, Der kolloide Schwefel, Upsala, 1913. 
2 For some remarks regarding highly dispersed foams, see WOLFGANG 
OstwaLp, Kolloid-Zeitschr., 1, 333 (1907). 


A2 COLLOID CHEMISTRY 


our definition of the colloids show that we may still speak of 
the state of aggregation of colloid particles. The individual 
particles of a typical colloid must certainly consist of a 
whole series of molecules. As we reach the more highly 
dispersed among the colloids the properties peculiar to any 
given state of aggregation must gradually disappear. The 
properties of solid, liquid and gaseous particles must, in 
other words, become more and more alike. This is a 
necessary conclusion from the principle of continuity ex- 
pressed in our diagram of the dispersed systems, for a 
molecular solution, for instance, of acetic acid in water, does 
not betray whether it was produced through the solution of 
solid, liquid or gaseous acetic acid in it. It is evident that 
we may expect to encounter interesting transition phe- 
nomena in this realm. 

These remarks will serve to indicate how broad is the 
field of the colloids when the different states of aggregation 
and their possible combination into dispersed systems are 
considered. Thus far we have dealt only with the sub- 
division of a material in a liquid dispersion medium. But 
the dispersion medium might, of course, be solid or gaseous. 
When all this is borne in mind, the following eight combi- 
nations become possible in which the dispersed material 
or dispersed phase is named first, the dispersion medium 
second. 

Solid + solid Solid + liquid (suspensoids) Solid + gas (smoke) 

Liquid + solid Liquid + liquid (emulsoids) Liquid + gas (fog) 

Gas + solid Gas + liquid (foams) 

It is important to emphasize that examples of coarse 
dispersions, of colloid dispersions and of molecular dis- 
persions are known to us under all these different headings, 
although the coarse dispersions and the molecular disper- 
sions are, for the most part, the more familiar examples. 

Many of the minerals, the very important alloys, the 
solid solutions of van’r Horr, etc., belong to the system 
solid + solid. As colloid examples of the class, I show you 
some blue rock salt (colloid sodium in sodium chlorid) and 


CLASSIFICATION OF THE COLLOIDS 43 


ruby glass (colloid gold in glass). Examples of the sub- 
division of a liquid into a solid dispersion medium may also 
be found in mineralogy, as in the occlusion, inclusion and 
crystallization waters found in all degrees of dispersion in 
rocks. Systems illustrative of the type solid + gas are 
meerschaum, pumice, lava and solutions of gases in metals. 
Gaseous colloids with a solid dispersed phase are seen in 
tobacco smoke, cosmic dust, the vapors of ammonium 
chlorid, ete. Systems of the composition gas + liquid are 
seen in fogs of all kinds, as in the fogs about the earth, and 
in the clouds of the sky. 

This list will, perhaps, impress you not only with the 
vastness of the general subject of the dispersed systems but 
with the extent to which these dispersions are of colloid 
dimensions. The modern concept of the colloid has served 
to bring together under one head many scattered elements. 
Where in the realms of physical chemistry could we formerly 
place the foams and the emulsions? These homeless and 
yet technically important structures are now not only 
properly cared for but are seen to be of the greatest signifi- 
cance in the science of colloid chemistry. 


$5. 


We come now to the main theme of today’s lecture. We 
are to show that transition phenomena mark our passage 
from the coarse dispersions into the colloids, and from these 
into the molecular dispersoids. I shall combine with this 
a more detailed discussion of the physical and chemical 
properties of colloid systems. To this end I shall demon- 
strate the mechanical, optical, electrical and physico- 
chemical properties of some colloid solutions while showing 
you at the same time how these change as we pass through 
the different degrees of dispersion. 'Today’s subject might 
be called the physico-chemical properties of dispersed systems 
and their variation with the degree of dispersion. 

Let us first consider some of the mechanical properties of 
dispersed systems. If you examine microscopically a fine 


44 COLLOID CHEMISTRY 


suspension of carmine particles in distilled water, you 
observe that the particles are in a state of spontaneous 
movement; they dance and rotate in an apparently irregu- 
lar manner, the individual particles following such paths as 
have been plotted in Fig. 19.1. These movements are not 
induced through an expenditure of light or heat energy nor 
are they dependent upon electrical or chemical changes. 
Moreover, all known dispersed systems show this so-called 
BROWNIAN movement whenever two conditions are satisfied. 
First, the dispersed particles must be sufficiently small. As 
a rule BROWNIAN movement does not manifest itself clearly 


Fic. 19.—“ Paths” of two particles in BROwWNIAN motion. 


until the particles have a diameter of 0.5 u» or less. Second, 
the dispersion medium must be sufficiently mobile to permit 
the movement. ‘The movements could not appear in solid 
glass, for instance. But if these two conditions are satisfied, 
all dispersed systems show BROWNIAN movement. It is 
observed, for example, in milk, in gas bubbles and very 
beautifully in smoke. It seems, therefore, to be a general 
property of dispersed systems and under constant conditions 
is apparently everlasting. BROWNIAN movement is ob- 
served in the liquid inclusions found in minerals which are 
certainly thousands of years old. 

How does this BRowNIAN movement change with changes 
in the degree of dispersion? Do we observe a BROWNIAN or 
similar movement in colloids and in molecular solutions? 


1 I was in the habit of concluding the lecture by demonstrating ultra- 
microscopic apparatus and with this, BRowNIAN movement. 


CLASSIFICATION OF THE COLLOIDS 45 


I have already told you that BROWNIAN movement of micro- 
scopically visible particles is observed only when these are 
highly dispersed. The intensity of the movement increases 
markedly as the microscopic particles decrease in size. Our 
concept of continuity would lead us to conclude that such 
movement must appear in colloids and molecular solutions 
also, and since the degree of dispersion is greater, the move- 
ment would occur much more rapidly here than in micro- 
scopic dispersions. There no doubt now comes to your 
mind the old and much-discussed belief that in all molecular 
systems, as in gases and liquids, the molecules are in a state 
of marked, even tumultuous, activity. The famous kinetic 
theory of gases and liquids is built upon this fundamental 
assumption. It can be shown by optical methods, which 
we shall discuss later, that spontaneous movement not only 
occurs in colloids, but is more intense in them than in micro- 
scopic dispersions. It has been possible to show that this 
greater velocity of BROWNIAN movement approximates the 
values calculated for the speed of molecules. Conversely, 
the laws which have been formulated for the kinetic move- 
ment of molecules hold also for the BROWNIAN movement 
of colloids and coarse dispersions if their degree of dispersion 
is duly considered. We shall return to this subject when 
we come to discuss the scientific applications of colloid 
chemistry. No physical chemist today questions the cor- 
rectness of the statement that this ‘spontaneous internal 
movement” is common to all dispersoids and that its 
intensity increases steadily as we pass from the coarse dis- 
persions, on the one hand, through the colloids, to the 
molecular solutions on the other. 


§6. 


Let us now consider another mechanical property of dis- 
persed systems. A particularly important qualitative char- 
acteristic of colloid solutions is their failure to diffuse and 
to dialyze. Typical colloids do not diffuse any more than 
do coarse dispersions. Are there transition systems which 


46 COLLOID CHEMISTRY 


occupy a position between the colloid and the molecular 
systems, or—and this would be a particularly pretty 
proof — can we make one and the same substance appear 
at one time in diffusible and at another in non-diffusible 
form? An experimental answer can be given to both these 
questions. We know many solutions which assume this 
intermediate position so far as diffusion is concerned. Many 
proteins, ferments, toxins, antitoxins and dyes, such as 
congo red, night blue, etce., show a hardly measurable but 
nevertheless definite diffusibility. Slight changes in the 
dispersion media suffice at times to impart to these transi- 
tion systems a well-marked diffusibility. Thus certain 
albumins do not diffuse into distilled water but diffuse 
readily into dilute salt solutions. The neutral salts dehy- 
drate the heavily hydrated colloid albumin particles, thereby 
increasing their dispersion and so their diffusibility.t All 
degrees of diffusibility are encountered in passing from the 
molecularly dispersed to the colloid solutions. 

But even one and the same substance in a given dispersion 
medium without any additions from the outside, may either 
diffuse or not, depending upon its degree of dispersion. 


1 Papers dealing with this subject hardly discuss the fact that the addition 
of a neutral salt or of alcohol to a hydrated colloid may bring about two 
totally different, antagonistic effects. First, addition of these foreign ma- 
terials increases the degree of dispersion by dehydrating the particles; through 
secondary agglomeration of the particles there then occurs a decrease in 
degree of dispersion which may end in coagulation. According to unpub- 
lished experiments of my own, this double effect is separable by proper 
methods, and explains, for example, the formerly unintelligible fact that 
protein solutions diffuse more readily into dilute salt solutions than into 
distilled water. See WoLraana OstTwaLp, Handbook of Colloid Chemistry, 
translated by Fiscumr, second English edition, 227, Philadelphia, 1919. Italso 
explains why colloid dyes like congo red upon the addition of salts first turn 
towards yellow and only later, shortly before precipitation, towards violet. 
The fact that large amounts of neutral salt must be present to accomplish 
the crystallization of proteins is also to be explained by the influence of the 
neutral salts in bringing about a decrease in the degree of dispersion of the 
hydrated colloid. It is presumable that the crystalline or vectorial forces 
of the particles will come into action best when the amount of indifferent so- 
lution medium bound to the particles and tending to inhibit their coalescence, 
is least. 


CLASSIFICATION OF THE COLLOIDS 47 


This was proved years ago by W. Ramsay’s pupils, H. 
Picton and §. E. Linprr, for the precipitates of arsenic 
trisulphid. In keeping with the law of von WrEIMaARN these 
authors obtained, from very dilute solutions, precipitates of 
arsenic trisulphid which were not only invisible under the 
microscope and passed a filter, but showed undoubted dif- 
fusibility. Similar observations, according to my experi- 
ence, may be made on the highly dispersed colloids of berlin 
blue and according to THe SvEDBERG on colloids of gold. 
In fact, in gold the connection between degree of dispersion 
and diffusibility seems so simple that the diffusion coefficient 
appears as inversely proportional to the diameter of the 
particles.1 This constitutes, moreover, a quantitative con- 
clusion derived from application of the kinetic theory to | 
these more coarsely dispersed systems. All these facts 
leave no room to doubt that diffusibility and therefore 
dialyzability increase progressively as the degree of dispersion 
increases, just as in BROWNIAN movement. 


§7. 


If we would discover examples of transition phenomena 
in the mechanical properties of coarse dispersions and of 
colloids we may study their behavior during filtration. We 
may recognize different degrees of dispersion as they will or 
will not pass through filters of a known porosity. To give 
you some idea of the estimated size of the pores in different 
filters, I show you the following table. 


SizE oF Pores IN FILTERS 


Filter paper No. 1450 (Schleicher and Schill).......... About 4.8 u 

Filter paper No. 598 (Schleicher and Schiill)........... About 3.3 yp. 
Prrmerveriicn Hiter Paper. ae ee eee eee eas About 3.3 yp. 

Filter paper No. 597 (Schleicher and Schiill)........... About 2.9 uw. 

Filter paper No. 602 hard (Schleicher and Schiill)...... About 2.2 u 

Filter paper No. 566 (Schleicher and Schill)........... About 1.7 p. 

Filter paper No. 602 extra hard (Schleicher and Schill).. About 1.5 u. 

Co ere k od OR C1 enn le a a a er a About 0.2 to 0.4 yu. 
CST TE SLE St a « ge ce dl ee a ne en aoa ae About 0.16 to 0.18 x. 


1 See Toe SvepBeRa, Zeitschr. f. physikal. Chem., 67, 105 (1909). 


48 COLLOID CHEMISTRY — 


In keeping with our definition, colloids would, therefore, 
be held back only by the fine porcelain filters. 

C. J. Martin, G. Maurirano, H. BecuuHo.p, and others 
have, however, taught us how to make filters which enable 
us to separate colloids from their dispersion media. We 
shall soon see, as a matter of fact, that filters may be pre- 
pared which will, in part at least, bring about a mechanical 
separation of dispersed phase from solvent even in the case 
of the molecular dispersoids. Such denser filtration media 
are found in different organic and inorganic gels. If ordi- 
nary filter paper, for example, is impregnated with collodion 
by the simple method which I have described,! these colloid 
or ultrafilters may be used to separate the dispersed phase of 
colloid solutions from the dispersion medium just as the 
materials of a coarser suspension may be separated from the 
‘solvent’? with an ordinary filter. Let me pour into a 
“spontaneous” ultrafilter thus made, a dilute solution of 
night blue, India ink, blue colloid gold, or a colophonium 
hydrosol (demonstration). As you observe there drips 
through only the colorless dispersion medium. In order 
to prove that the original highly colored fluids are really col- 
loid solutions and not coarsely dispersed systems I have set 
up a series of ordinary filters. As you see the different fluids 


1 Regarding simple ultrafilters see WoLFaane OstwaLp: Kolloid-Zeitschr., 
22, 72, 143 (1918); Kleines Praktikum der Kolloidchemie, 24, Dresden, 1920. 
The following recipe is a satisfactory one for the production of ultrafilters 
which filter ‘‘spontaneously,”’ in other words, under simple hydrostatic 
pressure. They are well adapted to demonstration purposes. A sheet of 
ordinary, smooth filter paper is fitted closely into a clean funnel and satu- 
rated with water, any excess of water being shaken out. Twenty to thirty 
ce. of a carefully warmed collodion solution (4 percent) are poured into the 
moist filter. A first collodion layer (the so-called “‘spongy”’ layer) is produced 
by turning the funnel about and allowing the collodion to spread evenly 
over the paper. Any excess is carefully poured away, care being taken that 
no drop remains in the tip of the filter. The whole arrangement is permitted 
to dry in the air some five to ten minutes during which time the stiffening 
filter is taken out of the funnel now and then. The collodion solution is 
poured a second time into the filter to produce a second layer, any excess of 
the solution being again carefully gotten rid of. After five to ten minutes 
of drying in the air the filter is submerged in distilled water. After remaining 
in this for about one-half hour it is ready for use. 


CLASSIFICATION OF THE COLLOIDS 49 


run through these in colored or turbid form. ‘The systems 
being filtered are therefore really colloid systems. In order 
to hasten ultrafiltration or to make filtration possible through 
still denser media the filters may be mounted in a suction 
flask (Fig. 20) and connected with a water pump; and to 
obtain larger filtration surfaces porce- 
lain funnels may be used. I have on 
the table several styles of such ultra- 
filters, useful for various purposes. 
We can so vary the permeability 
of the ultrafilters through the ad- 
dition of various substances or by 
changing their concentration that 
from a given colloid we may ob- 
tain fractions differing from each 
other in the degree of their disper- 
sion. Concentrated gels hold back 
even the most highly dispersed col- 
loids. If we use inorganic gels of 

the type of berlin blue made by bee aencicanors 
mixing together highly concentrated solutions, we obtain 
the so-called semipermeable membranes used in osmotic 
experiments. These jelly-like precipitates may be so im- 
permeable, as in the case of copper ferrocyanid, that they 
will not give passage even to many dissolved molecules. 
We can make use of this property not only to bring about 
changes in the concentration of molecular solutions in 
which the osmotic pressure assumes the réle of filtration 
pressure, but to bring about a separation of the solid salt 
in highly concentrated solutions. Thus, according to the 
physiologist C. Lupwie, a fairly concentrated solution of 
sodium sulphate begins to crystallize when a piece of dried 
pig’s bladder is introduced into it, for only water and not 
salt diffuses into this concentrated gel. All this again serves 
to show that we pass by easy steps from ordinary filtration 
through ultrafiltration to osmotic or “‘superultrafiltration” 
(P. P. von WEIMARN). 





50 COLLOID CHEMISTRY 


Ss. 

When we consider the optical properties of colloid systems 
We again encounter a large number of beautiful and inter- 
esting transition phenomena. Perhaps the most general 
optical phenomenon encountered in dispersoids is that, of 
optical heterogeneity, or turbidity. A dispersed material 
and the dispersion medium will ordinarily hardly be expected 
to show the same coefficient of refraction. A ray of light 
passing through the system will, therefore, not be able to 
do so undisturbed. This is the scientific explanation of 
turbidity, so well shown by coarse and microscopic disper- 
sions of all kinds. J need but remind you of the white color 
of quartz suspensions, of milk and of foam. But a large 
number of colloids also appear turbid if properly studied, 
as these colloid metallic sulphids, this bluish-black colloid 
gold, ete. 

We can best perceive slight turbidities by viewing a 
solution against a black background, in other words, by 
light coming chiefly from one side. How much unilateral 
lighting aids us in recognizing slight turbidities is familiar to 
you from observing dust particles in a ray of sunlight. 
When a pencil of bright light is thrown into a darkened 
room we not only see the bright cone but in it a large number 
of dust particles which escaped us when the light came from 
all sides. This method of demonstrating fine turbidities was 
used even by Farapay to prove the disperse nature of 
solutions of colloid gold. In this way he demonstrated 
the disperse nature of red gold which ordinarily seems 
entirely clear. The method was used in more extensive 
fashion by J. TyNDALL, in whose honor we call the light 
cone observed in dispersoids when illuminated from one side 
only, the TYNDALL cone. 

The majority of all typical colloids shows a Tyndall cone. 
Since this is a matter of much interest I shall demonstrate 
it to you (demonstration). 

We have here an arc light from which we obtain a narrow 


CLASSIFICATION OF THE COLLOIDS ol 


pencil of light which passes into this vessel with parallel 
sides filled with distilled water. You observe, I hope, 
nothing but a slight glow in the water. If the water were 
absolutely pure, if it did not contain even the slightest 
traces of dust or air, and were I able to shut out all reflection 
from the walls of the vessel, you would see nothing at. all. 
It is experimentally possible to produce water which to the 
naked eye is thus “optically empty.” Let me now pour 





Fic. 21.—A TYNDALL cone. 


into this vessel a few cubic centimeters of a brown solution 
of colloid silver, which as you saw before, is also perfectly 
clear to the naked eye. As the two liquids mix, an intense 
greenish-white cone of light flashes into view (Fig. 21). 
This is the famous TYNDALL cone and is due to the optical 
heterogeneity of colloid solutions. 

I could take up one after the other of the colloid solutions 
on the table and in almost every one of them show you this 
TYNDALL effect. 


52 COLLOID CHEMISTRY 


There are, of course, colloids which show it but little, 
such as blood serum,! alkali albuminates, freshly prepared 
silicic acid and very pure congo red solutions. These 
colloids belong to that second class of emulsoids to which I 
have already called your attention and which we shall 
discuss in detail in the next lecture. These colloids are 
characterized by their great hydration or solvation. Their 
particles have taken up a large amount of the dispersion 
media; in fact, they may at times consist chiefly of this. 
This explains why such colloids show the TYNDALL effect 
but weakly. To change the direction of a light ray it is 
necessary that a distinct refraction difference exist between 
dispersed phase and dispersion medium. But if the colloid 
particles are largely built up of the dispersion medium itself, 
the optical difference between combined and uncombined 
dispersion medium is but slight. This is why typical 
colloids and even coarse dispersions do not necessarily 
appear turbid. Coarsely dispersed powdered glass in 
Canada balsam of the same coefficient of refraction, for 
instance, does not appear turbid. One must be careful, 
therefore, to avoid the common mistake of concluding that 
a material is highly dispersed just because it is not turbid.’ 


$9. 


To what degree of dispersion may a material be carried 
and still show this TynpaLL phenomenon? And what 
changes does it show as we pass over into the field of molec- 
ularly dispersed solutions? In order to answer these ques- 
tions we must enter, for a moment, upon the theory of these 
turbidities. As we ascend the dispersion scale, optical 
heterogeneity becomes attributable more and more to 


1 According to F. Borrazzi fresh blood serum is practically clear opti- 
cally; see WINTERSTEIN, Handbuch der vergleichenden Physiologie, 1, 145. 

2 A silicic acid solution showing practically no TyNDALL phenomenon has 
been observed by C. O. WEBER, Chemistry of India Rubber, 3rd edition, 
74, London, 1909. 


CLASSIFICATION OF THE COLLOIDS 53 


optical causes other than the mere lateral deviation of 
light rays observed in coarsely dispersed systems. While 
in the latter the turbidness is chiefly attributable to refrac- 
tion, that in the most highly dispersed systems is due to 
diffraction. When the diameter of the dispersed particles 
falls below that of the length of the light waves illuminating 
them, refraction in the ordinary sense of the word can no 
longer take place. Instead, a diffuse dispersion of the light 
in all directions takes place. This occurs even in colloids, 
for colloid particles have already a diameter of but half a 
wave length or less. The TyNDALL phenomenon as ob- 
served in colloid solutions is therefore really due to light 
dispersion. It follows from the nature of this dispersion 
that when mixed light is used the shorter rays are bent more 
than the longer ones. The blue, violet and ultraviolet rays 
of a TYNDALL cone will, therefore, be bent more than the 
yellow and red rays. This results in that play of colors 
known as opalescence, to which we shall return in a moment. 
Furthermore, it is clear that the short waves will still be bent 
by particles too small to bend, for instance, the blue, yellow 
orredrays. This isa matter of much interest to us. When 
the ‘TYNDALL method is so refined that we are enabled to 
perceive not only mixed blue or violet light but ultraviolet 
light, then 2 becomes theoretically possible to recognize a 
turbidity even in molecularly dispersed systems. Just as col- 
loid particles may interfere with the longer waves of visible 
light, molecules may effect a disturbance in the shorter 
ultraviolet rays... One way of refining the TyNDALL method 
is to employ photography, which you know to be particularly 
effective in proving the presence of the chemically active 
ultraviolet rays. I have myself observed that distilled 
water which is optically clear to the naked eye shows a 
marked TyNDALL cone photographically. But this is lost 
on interposing between water and camera a thick glass plate 


1 Regarding TyNpALL cones due to ultraviolet and even shorter light 
waves, see WOLFGANG OsTWALD, Koll.-Zeitschr., 13, 121 (1913). 


54 COLLOID CHEMISTRY 


which absorbs most of the ultraviolet rays. By employing 
ultraviolet TyNDALL cones, a number of investigators have 
recently been able to demonstrate by photographic methods 
the existence of turbidities in systems of various kinds! 
not directly discernible by the senses. 

But how about experimental proofs of the existence of 
such extremely fine turbidities in the transition systems 
between colloids and molecularly dispersed solutions; or 
of their existence in the latter? It is self-evident that 
experiments to this end belong to the most delicate in the 
whole realm of physical optics. First to be considered is 
the presence of impurities, the presence, in other words, of 
‘‘optical dust,’? in the liquids to be examined which tends 
to enter every time that they are manipulated and which 
explains, for example, the paradoxical finding that the 
amount of such optical dust is often increased instead of 
diminished through filtration, distillation, etc. The Bel- 
gian investigator, W. SPRING, has devised ingenious methods 
to clear such systems of optical dust and has observed the 
purified liquids to be optically clear when observed directly. 
But photographic study of these liquids by M. Lz Branco 
and W. Kanaro still showed the plates to be affected, — 
there still existed, in other words, delicate optical impuri- 
ties. Furthermore, W. Sprina, Lopry pE Bruyn and 
others have shown that many of the molecularly dispersed 
solutions, like concentrated cane sugar, cannot be made op- 
tically clear by any method whatsoever, in contrast to other 
molecularly dispersed systems which, after purification by 
sumilar methods, no longer show subjectively a Tyndall cone. 
Evidently, there exist here the transition phenomena which 


1 This greater sensitiveness of a TyNDALL cone to photography probably 
explains the discrepancies between the studies of W. Kanaro [Zeitschr. f. 
physik. Chem., 87, 257 (1914)] and W. Sprina. While. the latter succeeded 
in obtaining water and various salt solutions in a form optically empty to 
the human eye — a possibility which every microscopist is able to corrobo- 
rate — the former could confirm these negative findings of Sprine only in 
part when he used photographic methods. 


CLASSIFICATION OF THE COLLOIDS 55 


interest us, for a concentrated cane sugar solution shows 
itself by other methods to approximate a colloid solution.1 
P. Wousxk1 working in the Physico-Chemical Institute of 
the University of Leipzig? has lately prepared, by methods 
of ultrafiltration employed in the absence of air and under 
otherwise extremely delicate laboratory conditions, con- 
centrated cane sugar solutions which in a thickness of 45 wu 
showed no TYNDALL cone subjectively. But this does not 
yet prove that turbidities are non-existent in molecularly 
dispersed systems. Liquid sheets so thin (45 » = 0.045 
mm.) might be clear while thicker ones might show a tur- 
bidity, especially if investigated by indirect, for example, 
photographic methods. 

A striking example which illustrates the importance of 
the thickness of the layer in bringing about a turbidity is 
furnished by the earth’s atmosphere. No one, under a blue 
sky and upon a mountain top, would consider air a turbid 
medium or would be able to demonstrate a turbidness 
in a layer 1/500 of a millimeter thick. And yet the purest 
air when viewed in its entire thickness shows the optical 
properties of a turbid medium — properties which inter- 
estingly enough are attributed by Lorp RayLuicH and other 
physicists, not to the presence of dust and water par- 
ticles, but to the dispersing effects of the air molecules them- 
selves. The famous Polish investigator M. von SmMoLu- 
CHOWSKI has, as a matter of fact, observed that carefully 
purified air shows a slight TyNpALL phenomenon even in 
laboratory experiments, —a turbidity which he, too, de- 
clares molecular. We are face to face here with compli- 


1 A discussion of transition phenomena as observable in concentrated 
cane sugar solutions may be found in WoLFGANG OstwaLp and K. MUNDLER, 
Koll.-Zeitschr., 24, 11 (1919). 

2 See M. Le Buanc and P. Woxskt, Ber. d. sichs. Ak. d. Wiss., 72, 24 
(1920); P. Wousxk1, Kolloidchem. Beihefte, 13, 137 (1920). 

83M. Von Smo.tucnowskI, Bull. Acad. Cracowie, 218 (1916); R. Furs, 
Schwankungserscheinungen in der Physik, Vieweg’sche Samml., No. 48, 61 
(1920). 


56 COLLOID CHEMISTRY 


cated questions. Not only the presence of impurities 
but the methods used, the thickness of the layer, the in- 
tensity of the illumination — all these and other factors 
need to be considered when we try to say that molecularly 
dispersed systems are turbid or not. At the present time 
we can only say that facts,! exist which suggest that pure, 
molecularly dispersed systems may show an optical hetero- 
geneity especially when observed in thick layers and with 
light of short wave length but that conclusive proof is 
difficult. Our concept of continuity, however, convinces 
us that such experimental proof will some day be forth- 
coming. 

This concept of continuity leads us further.2 It is clear 
that we may employ rays even shorter than those of ordi- 
nary light of the ultraviolet light to prove the existence of 


1 The optically clear, concentrated solutions of cane sugar, citric acid, 
etc., prepared by P. Wo.uski show a property which is never observed in 
dilute solutions or water, and one which speaks strongly for an optical hetero- 
geneity in these systems. When dust particles, purposely introduced into 
these solutions, are observed ultramicroscopically they are found to be sur- 
rounded by a remarkably large bluish-white field of light which is never ob- 
served in water, for example, and which is so intense that it often lights up a 
large part of the whole visual field. One is inclined to attribute the fact to a 
“secondary TyNDALL effect”’ in which the source of light is derived from that 
reflected by the dust particles. 

2 WoLFGANG OstwaLbD, Koll.-Zeitschr., 18, 121 (1913) from which I quote: 
“This refraction of RONTGEN rays should be used not only for the study of 
the symmetrically arranged discontinuities observable in crystals (as has 
been done by M. Lave and his co-workers) but for the analysis of the irregu- 
larly arranged or diffuse refractions observable, for example, in homogeneous 
liquids, molecular solutions, etc. It will broaden our knowledge of turbidity 
and of radiation effects in dispersed systems when this method is used and 
will develop knowledge beyond that now obtainable through the use of ultra- 
violet light. A RoOnraEmN ‘TYNDALL cone’ would make accessible to optical 
investigation degrees of dispersion lying beyond the reach of the normal 
TYNDALL cone and the ultraviolet TynDALL cone, the microscope and the 
ultramicroscope. It would be of interest if this question were investigated 
experimentally.”” The remarkable work by P. DmByr and W. Scumrrer 
who have actually used this idea in their study of benzene, no doubt without 
knowledge of the above remarks, did not appear in the Physikalische Zeit- 
schrift until 1916, 


CLASSIFICATION OF THE COLLOIDS 57 


fine optical heterogeneities, for example, RONTGEN rays, 
which are but 0:04 to 0.06 uw» long and which, according to 
the observations of C. Barkua and others, can also be 
deviated from their course. For the solution of this prob- 
lem the investigations of M. Laun, the two Braags, P. 
DeEBYE and their co-workers are important, who succeeded 
in photographing the refraction pictures of RONTGEN rays 
passed through crystals and in this way obtained pictures 
consisting of tiny spots of light arranged in a manner and 
at distances from each other which in large measure corre- 
sponded with the arrangement of the atoms in the crystal. 
Each of the spots upon the photographic plate was made 
by a concentrated pencil of RONTGEN rays and showed an 
intensity dependent upon the spacial orientation of the 
molecules in the crystal.!. Even when RONTGEN rays are 
passed through ‘‘homogeneous’”’ liquids like benzene P. 
DeBsYeE and W. SCHERRER observed diffraction phenomena 
permitting of deductions regarding the size and shape of 
the benzene molecule. There is therefore in this region of 
the highly dispersed systems also striking agreement be- 
tween theoretical deduction and experimental finding. 

One can hardly imagine a more perfect and continuous 
series of phenomena for proving the continuity of the 
different classes of dispersed systems than that represented 
by the microscopic turbidity of coarsely dispersed systems, 
the visible TYNDALL cone of typical colloids and concentrated 
molecular dispersoids, the invisible ultraviolet TyNDALL 
cone, and finally the RONTGEN ray TYNDALL cone of sys- 
tems alleged to be homogeneous. 


$10. 


Permit me to return for a moment to the TYNDALL 
phenomenon as observed in typical colloids. It is of much 


1 For details, see, for example, the collective presentation of F. RInnz, 
Die Naturwissenschaften, Nos. 17 and 18 (1916); No. 4 (1917). 


58 COLLOID CHEMISTRY 


interest, for it illustrates a principle of great importance in 
the analytical methods of modern colloid chemistry. As 
you know, when observing dust in the sun, we occasionally 
see particles become visible temporarily which are so small 
that we miss them ordinarily. If we watch closely, we note 
that these particles are surrounded by a luminous ring or 
halo similar to that observed along the edges of an opaque 
object when viewed against the setting sun. As the object ' 
viewed against the light becomes smaller, its edges become 
contracted and the visual image no longer corresponds 
accurately to the geometrical figure. When the particles 
become sufficiently small, the figure disappears entirely and 
its place is taken by a single luminous point. Similar 
phenomena are observed under the microscope when we use 
dark ground illumination. The arrangement corresponds to 
a viewing of the TYNDALL phenomenon against a dark back- 
ground. It is important to remember that particles will 
show this diffraction phenomenon even when smaller than 
a wave length of light. The limits of ordinary microscopic 
visibility, or, to put it more accurately, the limits for obtain- 
ing a correct geometrical picture are set, as I told you, by 
the length of the light waves. We can still, however, obtain 
diffraction pictures or diffraction spots of particles which 
are smaller than the length of a light wave. 

This method of demonstrating optically the presence of 
individual particles less than a wave length in diameter, by 
utilizing the principles of diffraction, but with sacrifice of 
the geometrical image, we call ultramicroscopy. Since col- 
loids are by definition dispersed systems in which the dis- 
persed particles have a diameter of less than a light wave, 
they may be rendered visible by using dark ground illumina- 
tion. This was accomplished for the first time by the two 
German investigators, H. SrEDENTOPF and R. ZsiaMonpy, to 
whom we are also indebted for important developments in 
ultramicroscopic methods. The importance of ultrami- 
croscopic methods in rendering visible the individual par- 
ticles in colloid systems and in thus proving the gradual 


CLASSIFICATION OF THE COLLOIDS 59 


transition of coarsely dispersed particles to those of colloid 
dimensions is self-evident. 

You obtain a good idea of the principles of ultramicroscopy 
if you imagine yourself looking at a TYNDALL cone with a 
lens or a microscope. The Belgian investigator W. SPRING 
many years ago used a magnifying glass on TYNDALL cones 
and the beginning of our present day ultramicroscopy may 
be seen in the arrangements for dark ground illumination so 





Fic. 22.— A simple arrangement for ultramicroscopy according to 
R. ZSIGMONDY. 


long used by bacteriologists and students of diatoms. We 
can, of course, use high powers of the microscope to accom- 
plish the optical analysis of a TyNpALL cone. The brightest 
spot of a small TyNDALL cone is then thrown just below the 
objective of a microscope, as shown in Fig. 22. It would 
take us too far afield were I to discuss the details of construc- 
tion of ultramicroscopes or to tell you of the many observa- 
‘tions that have been made with them. I shall only point 
out that such suspensoids as the colloid solutions of the 
metals yield varied and often extraordinarily colored ultra- 
microscopic pictures. The TynpALL cone produced by a 
solution of colloid gold is filled with innumerable brilliant 


60 COLLOID CHEMISTRY 


-points showing beautiful BROWNIAN movement. On the 
other hand, such things as proteins and certain dyes do not 
analyze into ‘“‘ultramicrons.” This is not always because 
the particles are too small but because, as previously stated, 
they are so highly hydrated that their coefficient of refrac- 
tion is not very different from that of their dispersion media.! 
These facts can be better demonstrated than described and 
so I must ask you to wait for the experiment which I shall 
present at the end of this lecture.2~ What, now, are the 
limits of visibility of particles ultramicroscopically? This 
question is of interest in connection with our concept of 
continuity. Suffice it to say that such limits are in high 
degree dependent upon the intensity of the illumination. 
By using light from an arc or from the sun we may still 
establish the existence of particles having a diameter of a 
few millimicrons. Of course the photochemical effects of 
the light are often so intense in such investigations that they 
are carried out with great difficulty. 

In connection with our concept of continuity, it is a matter 
of importance that R. Zstamonpy was able to produce rose 
colored colloids of gold which were so highly dispersed that 
they could not be analyzed under the ultramicroscope even 
when direct sunlight was used, a behavior in keeping with 
the previously discussed fact that these colloids show also a 
distinct tendency to diffuse. 


$11. 


Of much interest is the color of colloid systems. One of 
the simplest of the questions under this heading is that of 
their opalescence, or, differently expressed, the ‘‘color of the 


1 This mistake of concluding from negative ultramicroscopic findings that 
the material in hand is therefore necessarily a molecularly dispersed one is 
still made. 

2 I was in the habit of demonstrating at the end of this lecture such 
ultramicroscopic apparatus as circumstance provided, the nature of the 
demonstration being determined by the number in the audience and its 
interest. For a detailed description of ultramicroscopic pictures, the reader 
is referred to the text-books of colloid chemistry. 


CLASSIFICATION OF THE COLLOIDS 61 


colorless colloids.” All sorts of solid, liquid and gaseous 
colloids are bluish or violet when viewed against a dark 
background; or yellow and red when we look through them. 
You need but look at these solutions of gelatin or albumin, 
at this colloid mastic (prepared by pouring an alcoholic 
solution of mastic into water), at this milk glass, and at this 
white opal? (demonstration). The greatest example of a 
gaseous dispersoid exhibiting opalescence is seen in the sky. 
When we look at a cloudless sky against the dark back- 
ground of space, this heavenly dispersoid looks blue; but 
when we look through it against a source of light (as against 
the sun when it is coming up or going down) we find it yellow 
or red. The cause of this opalescence is to be found in the 
fact that the longer yellow and red rays are less disturbed 
and bent in a highly dispersed system than are the shorter 
violet and blue rays. 
To show you that not only ‘yellow and blue colors may 
appear in dispersed systems composed of two substances in 
themselves colorless, I present this flask of polymerized 
cinnamic ethyl ester. You note a beautiful greenish-red 
opalescence which would give way to a bluish-yellow were 
I to warm the mixture. The system consists, as shown on 
TYNDALL analysis, of a mixture of monomolecular ester and 
polymerized ester of which the particles have attained at 
least colloid dimensions. You may note similar color 
phenomena in the gelatinous colloid sodium chlorid pre- 
viously shown you. In fact, you may observe such 
CHRISTIANSEN Opalescence when you merely powder sodium 
chlorid very finely and suspend it in a mixture of benzol 
and carbon disulphid of practically the same coefficient of 
refraction as the sodium chlorid itself. It would take us too 
far afield to enter into the theory of these interesting 
phenomena.” : 


1 The play of colors in the opal is due in part only to opalescence, in part 
to the interference colors produced by thin plates. 

2 A detailed discussion of these CHRISTIANSEN colors and of related prob- 
lems will be found in a volume entitled, Light and Color in Colloids, which 
T hope to be able to publish soon. 


62 COLLOID CHEMISTRY 


Opalescence also varies greatly with the degree of dis- 
persion. Coarsely dispersed systems are but slightly opal- 
escent, while colloid systems are intensely so. So far as 
the opalescence of molecular dispersoids is concerned, we 
find that some investigators, like Lorp RAYLEIGH, assume 
that opalescence may still be produced through the dis- 
turbing effects of molecules. This is the case, for example, 
in certain gases, and it is held that at least a part of the 
opalescence of the sky may be dependent upon such .a 
selective bending of light rays by the molecules of the 


DISPERSED SYSTEMS 
COARSE DISPERSIONS—COLLOIDS—MOLECULAR DISPERSOIDS 
(100 to Lup) 


Increasing degree of dispersion ——»~ 





Opa ce tty 





Maximum 
in the colloid realm. 
Opalescence, Intensity of color, 
Catalytic activity, etc. 






Intensity -—> 






Fig. 23. 


atmosphere. Even when we grant this molecular opales- 
cence, the fact remains that its intensity decreases as we 
pass from the colloid into the molecular region. This is 
amply illustrated by the fact that most salt solutions and 
most gases are colorless unless viewed in enormous thick- 
nesses, aS presented by great masses of water or in the 
earth’s atmosphere. 

We note in this discussion a first illustration of the fact 
that the intensity of a property of dispersed systems may 
show a maximum, and this in the colloid realm. Previously 
we have only dealt with properties which either increased 
steadily with increase in degree of dispersion (as BROWNIAN 
movement and diffusion velocity) or such as decreased 
steadily (as the phenomena of heterogeneity) as we passed 


CLASSIFICATION OF THE COLLOIDS 63 


from coarse dispersions through colloids to molecular dis- 
persions. As we proceed, we shall find further illustrations 
of how certain properties of dispersoids may show a maxi- 
mum or minimum in the colloid realm, as indicated in 
Fig. 23. 

§12. 


The next property to be discussed, that of the intensity 
of color in colloid solutions, illustrates this. As is well 
known, such colloid salts of the metals as the sulphids may 
show so marked a color even in very low concentrations 


Intensity of color——————s»» 





1 2 3 4 5 6 7 
Degree of dispersion ———» 


Fic. 24.— Relation of color intensity of colloid gold to its degree of 
dispersion according to THE SVEDBERG. 


that it may be used for their qualitative recognition. ‘The 
coloration intensity of these colloids may at times be greater 
even than that of the aniline dyes. Thus if the coloring 
intensity of fuchsin is represented by the arbitrary value 
of 5, that of colloid iron hydroxid is about the same, while 
that of arsenic trisulphid is 100, and that of colloid gold 
about 2000 (THE SvEpDBERG). When the coloration inten- 
sity of a substance in different degrees of dispersion is 
studied, it is found to attain a maximum in the realm of 
colloid dispersion. 


64 COLLOID CHEMISTRY 


If we choose gold as an example, it is easily seen that a 
black, coarsely dispersed gold precipitate has less covering 
power than a solution of gold containing the same amount 
of metal in colloid form. When colloid gold is coagulated 
or precipitated, the solution becomes ‘‘decolorized.”” The 
small amount of black precipitate which falls to the bottom 
of the flask has again a minimal covering power. You 
will perhaps recall that in preparing colloid gold I started 
with a gold chlorid solution which was practically color- 
less; in other words, one which hardly showed the yellow- 
ish-brown color of the ion. From this colorless solution 
we obtained the intensely red and blue gold colloids. It is 
therefore certain that colloid gold has a more intense color 
than either coarsely or molecularly or ionically dispersed 
gold; in other words, a maximum is observed in the colloid 
realm. The accompanying Fig. 24, taken from THE SvEpD- 
BERG, illustrates quantitatively the appearance of this color 
maximum in the colloid region. 


$13. 


Not only is the intensity of colloid colors a noticeable 
fact, but their beauty and variety as well. I have already 
shown you red and blue gold, and by precipitating this 
metal with oxalic acid we can obtain green gold. Silver 
and platinum in the colloid state also show many different 
colors. Gold, silver and platinum may therefore be re- 
garded as panchromatic. 

I have here a number of photographic plates prepared by 
Litvppo-CRAMER’s methods, which show differently colored 
silver colloids in gelatin (demonstration).1 These were, as 
a matter of fact, made by his own hands. You observe 
that the plates are yellow, orange, red, violet and blue; 
and here I show you one that is green. When you first 
look at this plate it is greenish violet, but I need only dip 
it in water for a few seconds for you to see this color give 


1 These preparations were kindly given me by Drs. Ltprpo-Cramer and 
R. E. Linseeane. I should like here also to thank them for their kindness. 


CLASSIFICATION OF THE COLLOIDS 65 


way to a clear dark green (demonstration).! All these 
colors are the colors of colloid silver and one naturally asks 
why they differ so. Let me point out that we are again 
indebted to an American for a first study of this question. 
Carny Lema recognized and investigated the colors of col- 
loid silver and gold many years ago. 

The different colors of one and the same metal in a col- 
loid state are chiefly referable to differences in their degree 
of dispersion. ‘There is, of course, much room for differ- 
ences in degree of dispersion even within the realm of the 
colloid dimensions themselves which lie between 100 and 
1 yup, and it is in these finer differences that the explana- 
tion of the color changes must be sought. To prove this, 
I need but add some barium chlorid to this-red colloid 
gold, and stir the mixture (demonstration). The salt coag- 
ulates the gold; in other words, it changes the gold to a 
coarsely dispersed precipitate. <A first effect is a change in 
color. As you see, the mixture becomes violet. In a little 
while it will turn blue and then gradually become greyish 
black.2 The different colors of silver which I showed you 
are also to be explained through such differences in the sizes 
of the metallic particles, dependent upon the methods used 
in their preparation. 

The order in which the colors change from one to the 
other as the degree of dispersion changes seems also to be 
definite. As a rule, the most highly dispersed colloid 
metals are yellow or orange; in other words, they absorb 
violet and blue light. As the degree of dispersion de- 
creases, the color passes from yellow through orange to 
red, violet, blue and finally green. The absorption maxi- 
mum gradually moves toward the side of the greater wave 


1 See Ltpro-Cramer, Koll.-Zeitschr., 8, 240 (1911). 

2 These experiments are best made mak large quantities of the red wah 
gold [alcohol-gold, Wotraana OstwaLp, Kleines Praktikum der Kolloid- 
chemie, 2, Dresden, 1920] contained in two cylinders. Not too much of the 
barium chlorid must be added at once, otherwise precipitation in coarse form 
occurs immediately without the change to the blue color being clearly visible. 


66 COLLOID CHEMISTRY 


lengths as the degree of dispersion decreases.1. The same 
order is frequently observed in organic dyestuffs when 
the colors of any homologous series are studied. Yellow 
is usually the color of the chemically simpler members, 
while the dyes of greater molecular complexity in the same 
series are often blue and violet. 

I am indebted to my pupil R. AumerRBAcH? for a beautiful 
example of this relation between color and degree of dis- 
persion. This parallel-walled glass container illuminated 
from one side by the intense beam of light from a projection 
lamp contains a dilute solution of sodium thiosulphate 
(demonstration). I will add to it a few cubic centimeters 
of a dilute solution of phosphoric acid. As you know, the 
salt is decomposed by the acid with the liberation of sul- 
phur. At the concentrations employed the: sulphur sepa- 
rates off.in particles of colloid size. The particles will, 
however, grow gradually until a thick, milk-like mixture is 
obtained containing coarsely dispersed sulphur. During 
this gradual growth the system shows a surprisingly clear 
and beautiful series of different colors. Upon looking 
through the vessel the mixture is at first yellow but it soon 
changes to orange. If we wait a little while we will note 
a change to red. Later the mixture will become violet and 
then a beautiful and striking blue, passing, at times, to a 
blue-green. The color is then likely to fade, the system 
becoming non-transparent or ‘‘gray.’’ Please note that 
in the sulphur of this experiment we are dealing with a 
dielectric substance and that the changes in color observed 
obey the general law of relation of color to size of particle. 


1 For further details regarding this relation between color and degree of 
dispersion, see WOLFGANG OstTwALD, Kolloidchem. Beih., 2, 409 (1911), as 
well as my forthcoming monograph, Light and Color in Colloids. 

2 See R. AvERBAcH, Polychromie des Schwefels, Koll.-Zeitschr., 26, 239 
(1920). — A mixture, well suited to projection purposes, is the following: 
15 ec. 0.05 NaeS:O3 + [0.1 cc. H3sPO. (of a density of 1.70) + 4.9 ec. HO]. 
The whole run of colors from yellow to blue-green occurs within about 20 
minutes. The thickness of the fluid should be about 1 em.; the illumination 
as intense as possible. 


CLASSIFICATION OF THE COLLOIDS 67 


Our illustration is also interesting because no ‘“‘foreign”’ 
electrolyte was added so that all the changes observed can 
only have been due to the ‘‘spontaneous”’ increase in the 
size of the particles during their precipitation. 

What about transition phenomena to be observed in pass- 
ing from the colloids either in the direction of the coarse 





Fie. 25.— Variation in light absorption by colloid gold with change in its 
degree of dispersion according to Tue SvepBerG. The curve farthest to 
the right is that of the coarsest colloid. The curve farthest to the left is 
that of a molecularly dispersed gold colloid solution. 


dispersions or in that of the molecular systems? Follow- 
ing our definition of colloids we are not only justified but 
in duty bound to ask this question. I have already showed 
you how the colors of such colloid metals as gold change 
from red to violet and blue as the degree of dispersion de- 
creases. But what is the color of less colloid or coarsely 
dispersed gold? ‘Thin leaves of gold have in transmitted 
light a distinctly greenish color, and I have told you that 
gold may be obtained as a greenish precipitate. But what 
is the nature of the transition phenomena as we pass from 
the colloid to the molecularly dispersed or ionically dis- 


68 COLLOID CHEMISTRY 


persed gold? The most highly dispersed colloid gold thus 
far prepared is ruby-red or yellowish-red.1 Ionic gold in 
the presence of a colorless anion is distinctly brownish- 
yellow or orange. An accurate study of this transition can- 
not be made with the naked eye, but may be made spectro- 
scopically. In this way THr SvepBERG has shown that 
the absorption curves of colloid gold gradually approximate 
the absorption curves of tonic gold as the degree of colloid 
dispersion increases.” The curves of Fig. 25 illustrate this. 
The lowermost curve is the absorption curve of molecular 
gold chlorid. The uppermost one is that of a relatively 
coarse colloid gold. The curves between are those of 
colloids of successively greater degrees of dispersion. 

Let us consider next the colors of colloid silver. Here 
also the color passes from yellow through orange, red, vio- 
let and blue to green in the coarsest members of the series. 
But according to G. QuINCKE and others, the color of thin 
silver foil is also blue or green. The transition from the 
colloid realm to that of the coarse dispersions is therefore 
perfectly smooth. On the other hand, the most highly 
dispersed silver colloids are a transparent yellow or greenish 
yellow; in other words, they absorb chiefly violet and 
ultraviolet light. The more highly dispersed, the greater. 
is the transparency of these silver colloids. When prepared 
from very dilute solutions, or, differently expressed, under 
conditions which yield colloids of particularly high degrees” 
of dispersion, the yellow of these colloids becomes so faint 
as to be hardly recognizable. But this gradual disappear- 
ance of color in a highly dispersed colloid silver marks the 
passage from the color of the colloid into that of the silver 
ion. The latter, in the presence of a colorless anion, is 
colorless. 

Similar considerations hold for the colors of colloid plati- 
num. The most highly dispersed colloid platinum known, 


1 See THE SvepBERG, Koll.-Zeitschr., 4, 168 (1909); 5, 318 (1909). 
* See THE SvVEDBERG, l.c., as well as numerous other papers appearing 
in the Kolloid-Zeitschrift and the Zeitschrift fiir physikalische Chemie. 


CLASSIFICATION OF THE COLLOIDS 69 


that prepared by L. WouLER, is orange red, while the color 
of platinum salts is orange yellow. 

What has been said above holds not only for the colors 
of colloid metals but for those of the organic colloid dyes 
as well. I show you in Fig. 26 the absorption curves of 
indigo of different degrees of dispersion (THe SvEDBERG). 
In colloid form in aqueous solution indigo is blue, but, 


20000 |— 


10000 





400 
Fig. 26.— Absorption of light by indigo solutions of different degrees 
of dispersion. 


when molecularly dissolved, as in hot petroleum or chloro- 
form, it is red to violet. The lowermost curve (2) shows 
the absorption spectrum of an old and therefore relatively 
coarsely dispersed colloid; curve 1A, the absorption curve 
of an ordinary colloid; curve B, that of a molecular indigo 
in chloroform. You observe how the curves lie progres- 
sively higher as the degree of dispersion increases, while 
at the same time they move from the side of the shorter 
wave lengths to that of the longer.! 


1 In the experiments represented in the figure, the concentration of the 
colloids is only about half that of the molecular solution. The specific color 
intensity may not therefore be deduced from the figure. See THe SvEDBERG, 
Die Existenz der Molekiile, 51. 


70 COLLOID CHEMISTRY 


$14, 


The principle of continuity holds not only for the in- 
tensity and quality of the colors of dispersed systems but 
also for their optical rotation. Fig. 27 shows a series of 
curves which E. Navassart and I! obtained in a study 
of the optical rotation of tannin of different degrees of dis- 
persion. The ordinary aqueous solution of tannin, made 
by dissolving tannin in water, represents a polydispersoid, 
in other words, one in which there are particles of different 
degrees of dispersion. Most of the tannin is in colloid solu- 

70 


r 
50 : 
lalyzeq through fish p 
ladder 


40 


30 


Scag ane through parchment paper 


Normal in alcohol 


20 





10 
1 Zee 4 5 6 i 8 9: 10: .eiiesies 
Concentration 


Fic. 27. — Influence of degree of dispersion upon the optical rotation 
of tannin. 


tion yet some of it, as even GRAHAM knew, will pass through 
parchment paper, and is therefore in a state of higher 
dispersion. By using different grades of dialyzing mem- 
branes we can obtain different fractions of the aqueous 
tannin. Tannin is molecularly soluble in organic solvents. 
When the optical rotations of these different tannin solu- 
tions are compared, the coarsest tannin is found to produce 
the greatest rotation, the molecularly dispersed tannin the 


1 E, Navassart, Koll,-Zeitschr., 12, 97 Sate ; Kolloidchem, Beih., 5, 
299 (1914). 


CLASSIFICATION OF THE COLLOIDS Fil 


least. The behavior of two such tannin solutions, repre- 
senting the extremes of dispersion, is indicated in the upper- 
most and lowermost curves of Fig. 27. Between them 
are found the curves characteristic of tannin solutions 
which have dialyzed through parchment paper and fish 
bladder. These membranes have different sized pores, the 
fish bladder allowing larger aggregates to pass through than 
the parchment paper. As the order of the curves shows 
clearly, the specific optical rotation increases progressively 
in passing from the molecularly dispersed tannin to its 
colloid form. 

I believe you will agree with me when I say that a study 
of the changes in optical properties accompanying changes 
in the degree of dispersion proves in unequivocal fashion 
the correctness of the principle of continuity. 


§15. 


Not all the physical properties of dispersed systems have 
as yet been studied systematically from this point of view. 
But this is hardly to be wondered at, for this way of look- 
ing at the question is not yet ten years old.1. We strike a 
difficult and still ill-defined field in the general relations 
between degree of dispersion and the electrical behavior of 
colloids. Since it covers a series of properties which play 
a great réle in special colloid chemistry I shall touch upon 
it briefly. 

Most colloid systems, like most coarse dispersions, have 
an electric charge. We recognize it and its sense by send- 
ing an electric current through the system. When electri- 
cally charged, a colloid moves in an electric field — we 
observe the phenomenon of electrophoresis. The migra- 

1 WoLFGANG OsTWALD, Koll.-Zeitschr., 1, 291, 331 (1907); also Grundriss 
der Kolloidchemie, 1. Aufl. Dresden, 1909. Here was emphasized, so far as 
I know for the first time, the importance of the study of these transition 
phenomena and here too was suggested the curve-like nature of the vari- 
ations in physico-chemical properties with changes in degree of dispersion. 


Two years later appeared the observations of THe SveDBERG and his pupils 
which followed the suggestions of my earlier papers. 


i? COLLOID CHEMISTRY 


tion of a colloid is easily seen when the points of two wire 
electrodes are dipped into a drop of any dispersoid on a 
microscopic slide, or when the dispersoid is poured into a 
U tube and a current is sent through it. JI show you here 
two U tubes in one of which I have a mastic sol, in the 
other a colloid iron hydroxid (demonstration).1 The elec- 
tric current of the laboratory (110 volts) has been passing 
for about five minutes through both.2. The tubes have been 
placed in parallel. To the right is the positive pole, the 
anode. You observe how in the tube containing mastic 
the liquid is almost colorless about the cathode, while a 
thick white mass has collected about the anode. Col- 
loid mastic wanders to the positive pole. It is therefore 
negatively charged. A reverse behavior is observed in 
the iron hydroxid. The colloid has collected in thick flakes 
about the cathode. It is therefore positively charged. 

It is often possible to detect the sense of the electric 
charge of a colloid, even without the aid of a current, by 
very simple means. I have hung up here some strips of 
ordinary filter paper, the lower ends of which may be 
dipped into different colloid solutions. As you know, 
liquids tend to ascend such strips of filter paper through 
capillarity. I have here the colloid solutions of two blue 
dyes, alkali blue and night blue. Let me dip the ends of 
two filter paper strips into them, and even as I talk to you, 
you note the following: in the alkali blue the dye ascends 
together with the aqueous dispersion medium. ‘The water 
is slightly ahead, but nevertheless the dye follows close 
behind. In fifteen minutes it may have covered ten or 
more centimeters (demonstration). The night blue behaves 
totally differently. The water ascends far in advance of 
the dye. In other words, there is a separation of the dye 


1 Instead of these colloids, berlin blue or colloid graphite (negative) or 
night blue and alkali blue (positive and negative) may be used. 

* Since the speed of electrophoresis is roughly proportional to the differ- 
ence in potential, it is well to use currents of high potential and small 
amount. Overheating and the disturbances incident thereto must be care- 
fully avoided. 


CLASSIFICATION OF THE COLLOIDS | 13 


from the dispersion medium. After a quarter of an hour 
the dye will have concentrated at a point a little above 
the surface of the liquid, but it will not have followed the 
water (demonstration).! 

Let me next show you two experiments with different 
colloids of graphite (demonstration). The dispersed phase, 
the graphite, is the same in both, but the dispersion media 
are different. The two have been subjected to a ‘‘ capillary 
analysis.” This first dish contains colloid graphite in 
water (aquadag); the second, colloid graphite in mineral 
oil (oildag) diluted somewhat with ligroin.. The watery 
colloid has ascended with the water, but in the oily colloid 
the colloid phase has concentrated below, while the dis- 
persion medium has alone ascended the strip. 

According to F. Ficoter and N. Sauusom this difference 
in ascent is to be explained through the difference in the 
electric charge of the colloids. Negatively charged colloids 
ascend with their dispersion media, while the positively 
charged are held fast near the surface of the liquid and 
therefore separate from their dispersion media. The ex- 
planation of this 1s to be found in the fact that filter paper 
in contact with water assumes a negative electrical charge. 
Therefore when a positively charged colloid comes in con- 
tact with the paper the colloid becomes fixed electrostatic- 
ally. A negatively charged colloid, on the other hand, 
because of the similarity of the charges, goes by undisturbed.” 

An important and much overlooked fact regarding this 
electric charge of one and the same dispersed phase is its 


1 Particularly suitable concentrations are 0.2% night blue and 1.0% alkali 
blue. 

2 The experiment with colloid graphite in ligroin is not free from objec- 
tions. It is possible that the filter paper assumes toward ligroin and like 
substances a different charge than toward water. To explain the behavior 
under such circumstances, the second assumption would have to be made 
that the graphite in oil maintains its negative charge, which according to 
the experiments of G. QuINCKE is not true for the charge of sulphur particles 
in turpentine and in water [Wiedemann’s Annalen, 113, 513 (1861)]. A 
careful study of these relationships as well as of the whole capillary method 


74. COLLOID CHEMISTRY 


variability... As a rule, colloid metals and. sulphids, for 
instance, show a negative electrical charge, especially in 
aqueous dispersion media. Colloid iron hydroxid is usually 
positively charged; we are, however, familiar with nega- 
tively charged iron hydroxid sols. A striking illustration 
of the variability of the electric charge in one and the 
same colloid is furnished by the following experiment of A. 
LorrmRmMosgER. Either a positively or a negatively charged 
silver iodid may be obtained at desire by mixing a dilute 
solution of silver nitrate with one of potassium iodid. 
When the potassium iodid is poured into an excess of silver 
nitrate, we obtain a positively charged colloid. If we pro- 
ceed in a reverse way, pouring the nitrate into the iodid so 
that the latter is present in excess, we produce a negatively 
charged colloid. 

Then there are colloids which have only a very faint elec- 
trical charge. Protein and starch solutions very free of 
electrolytes belong in this class. Through the addition of 
aluminium sulphate, colloid gold can also be ‘‘discharged,”’ 
or ‘‘oppositely”’ charged, so that it either may not move 
at all in an electric field or may move toward the negative 
pole. 

The velocity of colloid migration practically equals that 
of ions and coarsely dispersed particles. But since accurate 
studies are still lacking on this relation of degree of dis- 
persion to speed of migration in an electric field, it may 
not yet be concluded that the latter is independent of the 
former.2. Even among the molecular dispersoids the larger 
and more complex ions migrate more slowly than those 
of FicHTER-SAHLBOM is much needed since the experimental facts do not 
always agree with the simple scheme outlined in the text above. See A. W. 
Tuomas and J. D. Garranp, Chem. Zentr., 1114 (1918); Woureane Ost- 
WALD, Kolloidchem. Beih., 10, 197 (1909); R. Km.umr, various papers in the 
Kolloid-Zeitschrift since 1920, ete. 

1 Insufficient consideration is given the fact that not only the amount but 
the sense of the electric charges in dispersed systems vary as greatly as the 
degree of dispersion. See WoLFGANG OsTWALD, Koll.-Zeitschr., 22, 79 (1918). 


2 See for example H. Frnunp.icu, ‘Kapillarchemie, 233, Leipzig, 1909 but 
also the newer views of G. von Hrvesy, Koll.-Zeitschr., 21, 129 (1917). 


CLASSIFICATION OF THE COLLOIDS 75 


of higher dispersion. It is reasonable to expect, therefore, 
that colloids will move still more slowly. 

When we inquire about transition phenomena, the dearth 
of available systematic investigations and the complexity of 
the phenomena make it impossible to establish here as simple 
and definite relations as were possible when discussing 
optical properties. Even the newer literature still argues 
the question of whether colloids are ‘‘ions’” or not. In 
answering this, much depends on what we understand by 
‘“ions.’’ If the term is used simply to cover all material 
carriers of electricity, then colloids and even coarsely dis- 
persed particles must be regarded as ions, for masses of elec- 
tricity are, of course, transported whenever any of these 
dispersed particles move in an electric field. But if by ions 
we mean only those material carriers of electricity which 
follow the laws of Farapay, as do the ions in salt solutions, 
if, in other words, the particles must always be the carriers 
of equivalent amounts of electricity, then colloid and 
coarsely dispersed particles are not to be counted among 
them; for to the present time no one has proved the validity 
of Farapay’s law for electrophoresis; and present knowl- 
edge indicates that the law does not hold in its ordinary 
form for colloids.t The influence of concentration upon 
electrophoric ‘‘conductivity”’ seems to be quite different in 
the case of the colloids and coarsely dispersed systems from 
that of a similar change in the case of the molecularly dis- 
persed electrolytes. There exist, however, some interesting 
analogies between the behavior of colloids and the properties 
of gaseous ions, in other words, those responsible for electrical 
conductivity in gases.2,_ Among these, too, are such as do not 
follow Farapay’s law. Here the influence of changes in 

1 For further details regarding discrepancies between the behavior of 
normal and of colloid ions, see, for example, Wotraane OstwaLp, Koll.- 
Zeitschr., 7, 132 (1910). The mathematics on page 152 of this article re- 
garding the similarities in the diffusion coefficients is, however, incorrect, 
since in the definition of diffusion coefficients an error was made in their 


values. 
3 See the preceding footnote. 


76 COLLOID CHEMISTRY 


concentration upon the conductivity of the gas is not the 
same as that observed in aqueous electrolytes. Just as in 
the case of colloid electrophoresis, we know gaseous systems 
in which predominate carriers of one kind, either positive 
or negative ions. In them electricity is carried predomi- 
nantly in one direction. The migration of gaseous ions under 
the influence of an electric current may be deviated by 
exposure to a magnetic field. This is a phenomenon which 
may be observed in colloids,! but not in normal aqueous 
electrolytes. These considerations would seem to indicate 
that further investigation of the analogies between electro- — 
phoric phenomena as observed in colloids and gases is likely 
to yield simpler and better material for the establishment of 
our principle of continuity than has thus far the study of 
the analogies between colloids and aqueous electrolytes. 


$16. 


Allow me in conclusion to touch upon‘a series of properties 
which again show the importance of the degree of dispersion 
upon the physico-chemical behavior of dispersed systems. 
It is a fact which has as yet received too little attention that 
such fundamental values as the maximum solubility, the 
freezing point and the melting point of a substance vary in 
marked fashion with the size of the particles of the material 
concerned. Thus, as far as known, the solubility of solids 
(in molecularly dispersed form) always increases with increase 
un the degree of disperston. The old experiments of G. Stas 
already suffice to show how true this is. Stas found the 
solubility of silver chlorid in the precipitated form (in other 
words, as particles of approximately colloid size) to be a 
hundred times greater than the solubility of this same sub- 
stance when granular or coarsely dispersed.2, This behavior 


1 See J. J. Kossonocow, Koll.-Zeitschr., 7, 129 (1910), as well as my re- 
marks following Kossonogow’s paper. 

* For details and for further examples see WoLraaAnac OstwaLpD, Hand- 
book of Colloid Chemistry, trans. by Martin H. Fiscuer, second English 
edition, Philadelphia, 1919. See also. the newer work of T. GLOVEZYNSKI, 
Kolloidchem. Beih., 6, 147 (1914) who could confirm the anaes of Stas 
only in part. 


CLASSIFICATION OF THE COLLOIDS ars 


is not peculiar to silver chlorid, but holds for all solid sub- 
stances. In analogous manner, the freezing point of water 
is lowered, not only through the presence in it of molecularly 
dispersed phases but simply by being allowed to freeze in a 
dispersed form, as when it is confined in capillary spaces. 
The water which has been allowed to soak into a clay sphere 
does not freeze until — 0.7° C., and wet filter paper does 
not stiffen until — 0.1° C. is reached. These are not simply 
phenomena of mere under-cooling, let it be noted. Looked 
at from the ordinary point of view, these facts simply indi- 
cate that determinations of the freezing point, were they 
carried out in capillary tubes, would always show too great 
values. But regarded from the point of view of the disper- 
soid chemist, they cannot help but suggest that the lowering 
of the freezing point in molecularly dispersed systems is also 
nothing but a “‘capillary’’ phenomenon; to him, the pres- 
ence of molecules and ions divides the solvent into many 
tiny capillary divisions through which the slight depression of 
freezing point noted, for instance, in the coarsely dispersed 
clay sphere, is compounded to attain the great values 
characteristic of molecularly dispersed solutions.! 

1 One might assume that the depression of the freezing point of a dis- 
persoid is proportional to its internal specific surface, in other words to the 
quotient of surface of the dispersed phase (per unit weight) and the volume 
’ of the dispersion medium. The value A mol. = 1.84°, the molar depres- 
sion of the freezing point of molecularly dispersed aqueous systems, would 
then represent the capillary depression of maximally dispersed systems 
(molecules and ions) containing one mol of the dispersed phase in the liter 
of water. In such maximally dispersed systems matters may then be sim- 
plified by substituting for the specific surface the concentration of the dis- 
persed phase, that is to say, the number of particles in the unit volume, 
since in such maximally dispersed systems all the particles have practically 
the same size. Such conditions obtain, however, only in extreme cases as 
in “infinitely” dilute solutions and not in concentrated solutions nor in 
such as are admixed with polymeric, associated or colloid particles. All 
the latter yield too small depressions of the freezing point as compared with 


the molecular solutions. In order to get the normal, maximal A value in 
such instances, a larger number of the large particles must be present in the 


unit volume. 
If, for simplicity’s sake, we assume a simple proportion to exist between 


specific surface and freezing point depression, the specific surface (Qm) of 


78 COLLOID CHEMISTRY 


The melting point of a substance also changes with the 
degree of its dispersion. It is lower as the degree of disper- 
sion is increased. Few of the organic chemists, who make 


molecular particles of an average diameter of 0.1 uu to the specific surface 
(Qk) of colloid particles having a diameter of 100 yup, if both were spheres, 
would be as about 5.108: 5.10° or as 1000: 1. See Woiraane Ostwatp, 
Handbook of Colloid Chemistry, translated by Martin H. Fiscumr, second 
English edition, 91, Philadelphia, 1919. In order to obtain the same specific 
surface, 1000 times as many colloid particles would therefore be needed in the 
same unit volume as when molecules are used. 

In order to obtain the normal or maximal depression of the freezing point 
in water, a gram-molecule (197.2 grams) of gold, or, roughly, about 200 grams, 
would therefore have to be molecularly dissolved in 1000 cc. of water. A 
molar solution of molecularly dispersed gold would therefore contain about 


20 percent gold; a molar colloid solution of gold possessed of the same spe- 


cific surface would have to contain oo in other words, 200 parts of 


gold to one part of water. This means that not until we dealt with a gold 
mud containing 99.5 percent colloid gold and about 0.5 percent water would 
we obtain a depression of the freezing point of 1.84°. A gold mud made 
up of particles 100 wu in diameter and containing about 5 percent of water 
would show a A value of about 0.1°. The same value would be shown by 
a mud containing particles 10 uu in diameter and about 33 percent of water. 
On the other hand, a 2 percent gold sol (which belongs to the most concen- 
trated of the gold sols that can be produced under normal circumstances) 
would, if its particles were 100 uu in diameter, be expected to show a de- 
pression of only 0.00018°; if the particles were 10 uy, a depression of 0.00184. 
These are values which correspond with those obtained experimentally. 

If we assume further that the capillary depression of the freezing point 
is independent of the specific chemical composition of the dispersed phase 
as taught in the classical solution theory, a dispersoid like wet filter paper 
which shows a A value of — 0.1° and is possessed according to H. BEcHHOLD 
of pores about 1 » = 1000 wu in diameter, would have a concentration of 
99 per cent solid material or a water content of about 1 percent. Sim- 
ilarly, a dispersoid of the type of the clay sphere of von BacumeEtTsEw, to 
which a pore value not exceeding 200 yu may be attributed, and which shows 
a A of — 0.7° would have a concentration of 99.3 percent or a water content 
of about 0.7 percent. As a matter of fact, much larger amounts of water 
have probably been absorbed in both these instances, even though the de- 
pression of the freezing point would theoretically be expected to be greatest 
when the water content is lowest. ‘The facts argue for the conclusion that 
at least a part of the pores and spaces in filter paper and clay spheres are 
decidedly smaller than the assigned values. This conclusion is in harmony 
with the observed facts. The presence, moreover, of molecularly dissolved 
impurities also tends, naturally, to give the observed depressions of the freez- 
ing point fictitiously high values. 

These remarks are merely intended to indicate the possibilities of a capil- 


CLASSIFICATION OF THE COLLOIDS 79 


such melting point determinations almost daily, know how 
great such lowering may be. P. Pawtow,! for example, 
found finely powdered salol, antipyrin, etc. (but in which 
the individual particles were still microscopically visible) 
to melt at a temperature 7° below that at which larger par- 
ticles did. F. Mu&issnmr,? on the other hand, states that 
this effect does not appear until the particles are submicro- 
scopic in size, so that the question of just where such an 
effect becomes appreciable is still debatable. 

Imagine such changes as observed in these relatively 
coarsely dispersed systems to be continued over into the 
realm of the colloids. It seems as if every property may in 
this realm assume a new value, wherefore we need not be 
surprised to find colloids exhibiting physico-chemical re- 
actions which the coarsely dispersed material shows not at 
all. 

According to what is known as WENZEL’s law, the reaction 
velocity of solids with liquids is proportional to the area of 
contact. It need not surprise us to find, in consequence, 
when the enormous surface presented by a colloid becomes 
available for reaction purposes — we shall return to this 
point in a later lecture — that colloid sulphur, for instance, 
acts as an energetic reducing agent for silver salts, a property 
which coarsely dispersed sulphur does not show at all;? and 
that colloid platinum still decomposes hydrogen peroxid 
when but one gram atom of the metal is present in seventy 
million liters of the dispersion medium (G. Brepia). We 
shall have occasion later to return to these catalytic effects. 


lary theory of these and allied phenomena. For the detailed development 
of such, quantitative experiments must first be carried out in coarsely dis- 
persed systems like capillaries. That measurements upon such coarsely dis- 
persed systems may be used after the fashion of the well known experiments 
of J. Perrin for the calculation of the constants of molecularly dispersed 
systems can also merely be pointed out here. 

1 P, Pawtow, Koll.-Zeitschr., 7, 37 (1909) where references to his earlier 
work may be found. 

2 F. Meissner, Zeitschr. f. anorg. Chem., 110, 169 (1920). 

$M. Rarro and A. Preront, Koll.-Zeitschr., 7, 158 (1910). 


80 COLLOID CHEMISTRY 


We need not, however, be surprised to find that still more 
highly dispersed metallic colloids prove less effective in 
decomposing hydrogen peroxid,! for we know that molecu- 
larly or ionically dispersed platinum (such as _ platinic 
chlorid) has but slight or no catalytic action. We observe 
again a curve showing a maximum and one of the same type, 
therefore, as previously noted in discussing the relation 
between color intensity and degree of dispersion. Coarsely 
dispersed and molecularly dispersed metals have little or no 
catalytic action, while those colloidally divided work most 
energetically. At a certain point in this middle region 
appears the maximum. 


$17. 


The subject of the relation between physico-chemical 
properties and degree of dispersion is very great and I could 
continue much longer with it, but perhaps I have tired you 
already. If so, I beg you to remember that the whole 
present day concept of the colloids stands and falls with the 
recognition of these relations. We are justified in regarding 
the colloids as special examples of the dispersed systems 
only if we are able to show that the properties of colloid 
systems blend gradually into the properties of the coarsely 
dispersed systems on the one hand, and into those of the 
molecularly dispersed on the other. This new concept is but 
a flimsy hypothesis as long as it cannot be experimentally 
proved. 

I hope that I have succeeded in bringing you such expert- 
mental proofs, even though, as I have emphasized, many 
parts of the subject have not yet been worked out in detail. 
The primary characteristic of a colloid is its special degree of 
dispersion. If this is true, then colloid chemistry becomes 
primarily not the science of the properties of a special group 
of substances, but that of the properties of a physico-chemical 


1 See St. Rusznyak, Zeitschr. f. physik. Chem., 85, 681 (1913). 


CLASSIFICATION OF THE COLLOIDS 81 


state into which any substance may be brought... Like the 
science of crystallography, colloid chemistry deals with a 
special physico-chemical state of matter. There exists, of 
course, a special colloid chemistry also which details the 
specific variations which any chemical substance may show 
when it happens to appear in the colloid state, just as 
remarks attached to a discussion of any chemical substance 
which are crystallographic in character may be found in our 
text-books. Formerly it was believed that colloid chemistry 
had spent itself when it had thus described in footnote 
fashion the colloid properties of chemical compounds. 
Today we insist upon the existence of colloid chemistry as 
an independent division of the physico-chemical sciences. It 
is the science of the colloidally dispersed state. The estab- 
lishment of this fact was, in brief, the main purpose of 
today’s lecture. : 


1 Since in recent years this conclusion has often been attributed to P. P. 
von WEIMARN I may be permitted to point out that it is mine; see Oppen- 
heimer’s Handbuch der Biochemie, 1, 853 (1908); also Grundriss der Kol- 
loidchemie, 1. Aufl., Dresden, 1909. 





+ 
1 
‘ 


1008 
THE CHANGES IN STATE OF COLLOIDS. 





THIRD LECTURE. 
THE CHANGES IN STATE OF COLLOIDS. 


Our previous lectures have dealt for the most part with 
the general physical chemistry of the colloid state, or of the 
dispersed state in general. I have tried to show you where 
the colloid systems fit into the great general scheme of the 
dispersed systems. ‘Today we shall discuss the colloidally 
dispersed systems more specifically. This does not mean, 
of course, that these possess properties for which there are 
no analogies in the neighboring realms of dispersion, for, as 
already emphasized, we pass gradually from the coarse 
dispersions into the colloids and through these into the 
molecularly dispersed systems— but a whole series of 
phenomena shows itself either most markedly or least 
intensely in the colloid realm and may in this sense be 
_ regarded as specific for the colloid state. These phenomena 
will absorb our chief interest today, and it will be our special 
problem to discover what special changes these typical 
colloids suffer when exposed to different external conditions. 
We shall, therefore, start with colloid systems, expose them 
- to different experimental conditions and see what happens. 
What we observe may justly be termed the special physical 
chemistry of the colloid state. _ 


§1. 

The considerations of our previous lectures again permit 
us to predict what must be the nature of a large number of 
these changes in the colloid state. Ignoring certain radical 
changes which have to do with gross destruction of the 
colloid itself by chemical means, what are the changes which 


a colloid may suffer? It may, first of all, undergo changes 
in its degree of dispersion. These changes may be limited 
85 


86 COLLOID CHEMISTRY 


to the region of the colloid realm itself, or they may extend 
beyond this into the realms of the molecularly dispersed or 
the coarsely dispersed. ‘These changes which occur within 
the limits of the colloid realm itself we call the internal 
changes in state, thus distinguishing them from the radical 
changes in state which take us beyond these limits. Of 
particular importance are the changes in degree of disper- 
sion occurring at the limit between coarsely dispersed and 
colloid systems. Decreases in degree of dispersion resulting 
in coarsely dispersed systems are designated coagulations. 
Liquid colloids are also known as sols; their coagulation 
products, as gels. Increases in degree of dispersion are 
designated peptizations, because they simulate the solution 
phenomena observed when solid proteins are acted upon by 
ferments. Obviously these two great classes of changes in 
state can be foretold. 
§2. 

Another great group of changes in state is connected with 
changes in the type of the dispersed phase in colloids. You 
will recall that there are colloids of the composition solid + 
liquid (suspensoids) and liquid + liquid (emulsoids). We 
shall consider these in detail today. It is an interesting fact 
that one and the same colloidally dispersed material may — 
vary in one and the same dispersion medium between solid 
and liquid; as we shall see later, the colloid particles may 
pass gradually from the solid to the liquid state. This is 
particularly true of the hydrated or solvated colloids, in 
which the dispersed particles carry about with them more 
or less of the dispersion medium in combined form. The 
process is analogous to the behavior of a piece of gum arabic 
which may show all transitions from a brittle solid to a liquid, 
depending upon the amount of water it has taken up. After 
I have showed you the experiments accompanying today’s 
lecture, you will be astonished at the great réle played by 
these changes in the type of the dispersed particles. The 
phenomena of gelation and of swelling belong under this 
heading. — | 


THE CHANGES IN STATE OF COLLOIDS 87 


§3. 


But we can foresee the existence of even a third class of 
changes. Were you ordered to sketch a colloid you would 
no doubt make it look something like Fig. 28, A. You 
would naturally assume that the colloid is distributed uni- 
formly through the mass. But closer study shows that this 
is only approximately true. Wherever the colloid comes in 


SUAS SNGMIG 1G, 


O 
O 
O 
O 
O 
O 





Fig. 28.— Diagram illustrating the concept of adsorption. 


contact with a surface (as the vessel wall or the air) it fails 
to maintain a uniform spacial distribution. At the surfaces 
of contact the concentration becomes different from that 
obtaining in the inner parts of the colloid mass. The 
surfaces contain either less or more of the dispersed phase 
than the rest of the system, as indicated in Fig. 28, B. 
Usually the colloid tends to concentrate in the surfaces. 
These changes in concentration are commonly designated as 
adsorption. It was the American giant, WILLARD GIBBS, 
who first pointed out that at the surfaces of dispersed sys- 
tems a different concentration was to be expected than 
prevails in the body of the dispersoid. Grpps did not, of 
course, either know or use this modern concept of the 
dispersed systems, but his deductions were of such general 
nature that they apply also to the special field which we are 
discussing. Adsorption phenomena are of great variety and 
play a great role in many different ways in the changes in 
state of colloids. 

It must be remembered that changes in degree of disper- 
sion, in type of dispersed phase and in its spacial distribution, 


88 COLLOID CHEMISTRY 


constitute by no means all the possible changes in state. 
Neither do these three groups of phenomena appear singly; 
they frequently appear in combination. The many com- 
binations possible and the innumerable resultant changes in 
state constitute the prime reasons for the great instability 
of the colloids. Even Granam said that rest never ruled 
in the colloid state. These facts render apparent why the 
scientific study of the changes in state of colloids is of such 
great interest and why such particularly complicated 
phenomena as those of life take place in colloid media and 
only in such. 


§4. 


But let us leave these theoretical considerations and return 
to the experimental. By what experimental methods may 
we study qualitatively and quantitatively the changes in the 
state of colloids? A first fact ever to be borne in mind is 
that changes in the state of a colloid always take time; 
differently expressed, they occur at definite velocities. They 
take time as do chemical reactions, but differ from these in 
that their end states are not so clearly defined. The end 
products of a chemical reaction are definite chemical com- 
pounds with constant properties; the changes in state of a 
colloid may halt when any degree of dispersion or hydration 
has been attained. The kinetic element, therefore, serves 
to characterize colloid changes in state even more markedly 
than it does the changes observed in the fields of “pure” 
chemistry. It is for this reason that the ideal method of 
studying colloid changes always proves to be a kinetic one. 
But it also follows that the characteristics of gelation, of 
swelling, of coagulation, etc., will never be marked by clearly 
defined ‘‘points.”” For instance, it is impossible to say that 
a protein solution coagulates when the. concentration of 
twenty percent ammonium sulphate has been reached. A 
“slight turbidness”’ is likely to be observable at ten percent; 
a “marked turbidness,” at fifteen percent; a “beginning 
precipitation,” at seventeen percent, etc. Nor will such a 


THE CHANGES IN STATE OF COLLOIDS 89 


protein be suddenly coagulated if its temperature is decreased 
a degree. ‘The expression of such findings in the terms of 
kinetics is far better.? 

In the case of the simpler changes in the colloid state, as 
when the degree of dispersion is merely altered, we may 
follow the changes in the size of the particles by employing 
the ultramicroscope, ultrafiltration, etc., and plotting the 
results as a velocity curve; or we may utilize the principle 
discussed in a previous lecture according to which every 
physico-chemical property of a colloid varies with its degree 
of dispersion. In this way I demonstrated to you the 
increase in the size of the particles during coagulation in a 
gold sol when calling your attention to the changes in color 
from red to blue. In complicated cases we still have recourse 
to such indirect methods. We measure, for example, 
changes in conductivity, in turbidity, in viscosity, etc., and 
then from their value parallel them with changes in the state 
of the colloid. Of course, we try to choose properties which 
are easily measured and of such prominence as to change 
with but slight alterations in state. The methods must, 
- moreover, permit a study of the changes in state without 
causing a destruction of the colloid itself. 

We observe changes in the internal state of colloids in 
great variety in such materials as gelatin, albumin, rubber 
or cellulose. We have called these colloids solvated emul- 
soids and will learn more of them shortly. One of their most 
conspicuous physico-chemical properties and one intimately 
associated with their internal state is their wscosity. No 
better example of its importance can be given than appears 
in the fact that the Farapay Society in 1913 devoted a 


1 So far as I know, I was the first to emphasize that changes in state should 
by definition receive kinetic treatment instead of being characterized by the 
customary ‘‘points.””’ See my Grundriss der Kolloidchemie, 1. Aufl., 267, 
- Dresden, 1909; also, Koll.-Zeitschr., 12, 218 and 246 (1913); Die neuere 
Entwicklung der Kolloidchemie, 18, Dresden, 1912. Following these sug- 
gestions of mine, there appeared the papers of H. Pann, H. FrReunp.icu, 
N. IsuizaKa, ete. See Koll.-Zeitschr., 11, 115 (1912); Koll.-Zeitschr., 12, 
230 (1913); Kolloidchem. Beihefte, 4, 24 (1912), where references to further 
papers will be found. 


90 COLLOID CHEMISTRY | 


whole meeting to its discussion.t Over a dozen colloid 
chemists spent two sessions on this special theme. Nor 
could I do better to illustrate the variety and importance 
of the internal changes in state of which a colloid is capable 
than by giving you an outline of some viscosity studies. 


§5. 

Anyone who has busied himself with dialysis or diffusion 
experiments soon discovers that there are two distinct 
classes of colloids among the many non-dialyzing, non- 
diffusing dispersoids with which he may work. Their 


Viscosity 





Suspensoids 


Concentration———> 


Fic. 29. — Diagram illustrating the relation between viscosity and 
concentration in different types of colloids. 
behavior is totally different. Especially ovident is the 
great viscosity of the one as compared with that of the 
other. Colloids of gold or of the metallic sulphids hardly 
alter the viscosity of their dispersion media. In low 
concentration these colloids are as mobile as their “‘sol- 
vents”. by themselves and oven in high concentrations 
their viscosity increases are only arithmetric in type. This 
is shown in curve A of Fig. 29. The behavior is char- 
acteristic of the solid + liquid type of colloids, the sus- 
pensoids. 
Colloids like gelatin which we said above are hydrated 
emulsoids behave entirely differently. They show enor- 
1 See Koll.-Zeitschr., 12, Heft 5 (1913). 


THE CHANGES IN STATE OF COLLOIDS 91 


mous viscosity values in even very low concentrations, 
and with every increase in the concentration of the colloid, 
the viscosity value mounts enormously, as shown in curve 
B of Fig. 29. As you know, gelatin in water shows all 
possible viscosity values from that of pure water to that 
of a solid jelly within the concentration realm of a two 
percent solution. We know other emulsoids which will 
show such variations from unit viscosity to infinity (the 
viscosity of solids) within even narrower ranges of con- 
centration. Castor oil soap, for example, is an almost solid 
jelly at a concentration of 0.1 percent, if a certain amount 
of alkali is present; and similarly striking figures may be 
cited for other organic colloids.! 


§6. 


Besides these great absolute and relative effects of 
concentration upon viscosity, other factors influence it, 
as temperature. As you know, the viscosity of molecules 
decreases steadily with increase in temperature. The 
viscosity of pure water, for instance, decreases some 
2 percent for every degree of rise in temperature between 
0° and 25° C. But in gelatin solutions such decrease in 
viscosity is enormously greater. In concentrated solu- 
tions or in gels the decrease is so great within certain tem- 
perature limits that phenomena are observed which seem 
analogous to the melting of homogeneous solids. Within 
a temperature range of but a few degrees, there occurs a 
fall in viscosity from the values characteristic of solids to 
those characteristic of fluids. It is an important fact, too, 
that the observed change is a progressive one and not 
such a sudden change in state as is observed, for example, 
in the melting of a crystal. 

High viscosity values are observed even in non-solvated 
emulsoids. Concentrated sulphur solutions, for example, 
may show a salve-like consistency. There exist many 


1 Further examples are detailed in the paper of W. Doutz, Koll.-Zeitschr., 
12, 73 (1913). 


92 COLLOID CHEMISTRY 


theoretical reasons why the liquid state of the dispersed 
phase should alone suffice to explain on a physical basis 
the great viscosity of the emulsoids.! 


§7. 


Besides concentration and temperature, other less drastic 
changes in external conditions alter the viscosity of sol- 
vated emulsoids. Simple shaking or repeated passage 
through a capillary, for instance, often suffice to decrease 
viscosity as has been observed in milk. The viscosity also 
falls when emulsoids are kept for a time at higher tempera- 
tures. In fact, time alone will effect viscosity changes. A 
colloid needs but to be left alone for a time in order 
that changes in its viscosity may be observed, — sometimes 
an increase, as in gelatin, at other times, a decrease, as in 
starch. 

Of course, the addition of extraneous substances, both 
electrolytes and non-electrolytes, suffices to influence the 
viscosity of colloids. I show you here a series of gelatin 
solutions to which various substances have been added 
(demonstration). The first tube contains a pure two and 
one-half percent gelatin. It is, as you see, a solid jelly 
which on hard shaking separates in pieces from the wall 
of the tube. The second tube contains the same concen- 
tration of gelatin, but to it have been added several per- 
cent of dry magnesium sulphate. The jelly is decidedly 
stiffer and does not break to pieces on hard shaking. The 
sulphates, citrates, phosphates, etc., all increase the vis- 
cosity of aqueous colloids of the type of gelatin, and of 
course, to a much greater degree than when added to pure 
water. The third tube again contains the same gelatin 
but this time some potassium iodid crystals were added 
to it. As you observe when I tilt the tube, the gelatin 
has remained fluid. Iodids, bromids, cyanids and certain 
chlorids decrease the viscosity when present in certain 


* See especially E. Harscuex, Koll~-Zeitschr., 7, 81, 301 (1910); 8, 34 
(1911); 11, 280, 284 (1912); 12, 283 (1913); 18, 88 (1913). 


THE CHANGES IN STATE OF COLLOIDS 93 


concentrations. The remaining tubes show the effects of 
some added non-electrolytes. Chloral hydrate and urea 
decrease viscosity. Alcohol in small amounts increases it. 
This problem of the effect of added substances becomes 
much complicated by the fact that one and the same sub- 
stance may either increase or decrease the viscosity, de- 
pending upon the concentration in which it 1s added. Thus, 
gelatin solutions to which acid or alkali is added in differ- 
ent concentrations show a minimum and a maximum of 
viscosity, while when chlorids are added several minima and 
maxima of viscosity may be noted. 

All these variations in viscosity correspond to changes in 
the state of the colloid systems themselves, such as changes 
in degree of dispersion, in type of dispersed phase or in 
degree of solvation. We know, for example, from ultra- 
microscopic and other means of investigation that in the 
ageing of dilute starch there occurs not only a decrease 
in dispersion but also a dehydration of the dispersed phase. 
The colloid particles not only give off a part of their water 
but they clump to form larger aggregates. Here a decrease 
in the viscosity parallels a decrease in dispersion and a 
passage from the liquid state toward the solid. Con- 
versely, the addition of alkali or acid in certain concen- 
trations may increase the hydration of certain colloids, 
as those of protein. Here the dispersed phase moves from 
the solid toward the liquid side, a change which again 
betrays itself by an increase in viscosity. 

With other colloids we are not yet sure what special 
internal changes in state are responsible for the extreme 
variations in viscosity. According to recent studies it 
seems probable, for example, that gelatin solutions while 
cooling develop an internal structure; in other words, the 
coalescence of the colloid particles yields aggregates that 
form fibrils, nets, etc. We shall return to this question 
later. The effect of adding certain substances, as salts 
in different concentrations, cannot as yet be connected in 
any satisfactory way with the accompanying changes in 


94 COLLOID CHEMISTRY 


degree of dispersion and of hydration. To get light here 
we need to supplement the general methods of physico- 
chemical investigation with the colloid-chemical ones of 
ultramicroscopy, ultrafiltration, ete. 


§8. 


Our discussion of gelation brings us into the field of the 
changes in state which occur within the limits defining the 
realm of the colloids themselves. Let us begin by asking 
what happens when a gelatin solution cools and sets. What 
are the internal changes which lead to the enormous in- 
creases In viscosity that are observed, for instance, when 
the originally liquid mass gradually changes into a solid? 
Let me illustrate the theory of gelation by an experiment. 

I have in this flask two liquids which at ordinary tempera- 
ture are only partially soluble in each other, namely, phenol 
containing some water, and water containing a little phenol. 
Even at a distance you observe that they scarcely dissolve 
in each other, for as soon as I shake the flask the mass of 
liquid shows the white color of an emulsion (demonstra- 
tion). The solubility of phenol and water in each other 
increases greatly with increase in temperature. To prove 
this I heat the mixture while continuously shaking it 
(demonstration). As soon as I have warmed the mixture 
to a temperature of 70° C. the emulsion clears; in other 
words, the amounts of phenol and water with which I 
started dissolve completely in each other. I could actu- 
ally add any amount more of either of the two, at this 
temperature, without their separating out. _ 

Above the “critical”? temperature, phenol and water are 
miscible in all proportions. For this experiment I have, 
of course, not chosen indifferent amounts of phenol and of 
water, but a concentration of about 36 percent of phenol. 
This represents the ‘‘critical’”’ concentration of phenol in 
water, the significance of which I cannot discuss further here. 

While I have been talking, the white emulsion of phenol 
and water has given way to a clear solution. We have 


THE CHANGES IN STATE OF COLLOIDS 95 


before us now a molecularly dispersed solution of the phenol 
in water. But the phenomenon to which I wish to direct 
your particular attention is to be observed when we begin 
to cool this system, either by shaking the flask in the air 
or by placing it under the water tap. You can see in ad- 
vance that as I reduce the temperature a separation of 
the system must again occur, for the solution phenomena 
that you have observed are entirely reversible. The solu- 
tion has now cooled somewhat, but if you observe closely 
you note at the same time that it has also changed in 
appearance. What was formerly a completely colorless 
liquid shows now a bluish yellow opalescence entirely iden- 
tical with the opalescence of an egg white solution, or a 
highly dispersed mastic colloid. The opalescence increases 
as the mixture cools. The similarity between the opal- 
escence in our phenol-water mixture and the opalescence 
of typical colloids compels the conclusion that we have 
before us a separation of the phenol in the water in colloid 
form. In fact, consideration of the problem compels the 
conclusion that such a colloid state must be passed through 
in the course of the separation in such a system; and it 
only becomes a question as to whether we are able experi- 
mentally to maintain or ‘‘stabilize”’ this colloid state, when 
reached, long enough to be able to examine it carefully. 
We begin with a temperature at which we have to do with a 
molecular division of the two substances in each other and 
end with a lower one at which we have to do with a coarsely 
dispersed, or even non-dispersed, mixture of phenol and 
water. Evidently somewhere between these two extremes 
we pass through the colloid realm and the opalescence of the 
mixture before you renders it probable that we have it be- 
fore us now. . Ultramicroscopic investigation furnishes 
direct proof that this opalescent critical fluad mixture is really 
one in which the divided phase has colloid dimensions. 

1 The analogy between critical fluid mixtures and colloid solutions was 
first pointed out by D. KonowatLow, Drudes Ann., 10, 378 (1905). With- 
out knowledge of his work I characterized these systems as emulsoids in 
1906. See Koll.-Zeitschr., 1, 335 (1907); Grundriss der Kolloidchemie, 


1. Aufl., 102, Dresden, 1909; W. VON Spienias Zeitschr. f. physik. Chem., 
75, 608 (1910). 


96 COLLOID CHEMISTRY 


Other, and, in part, startling analogies are discoverable 
between the properties of critical fluid mixtures and the 
properties of emulsoids, more particularly of the solvated 
emulsoids of the type of gelatin. Unfortunately, I cannot 
demonstrate all these to you. Critical liquid mixtures 
frequently show, for example, the properties of foaming, 
which are not observable in molecular solutions of these 

materials as obtained at higher temperatures. 


Butyric acid —water 





Temperature —> 





Fic. 30. — Viscosity of a critical fluid mixture. 


But more important still is the fact that with the ap- 
pearance of the opalescence there occurs also a remarkable 
increase in the viscosity of the mixture. As you know, the 
viscosity of every liquid increases on cooling, but in the 
case of critical fluid mixtures, this increase in viscosity is 
abnormally great in the regions of the critical temperature, 
and, after the opalescence region has been passed, the 
viscosity falls again even though the system, as a whole, 
is then at a lower temperature than before. 

If the viscosity of the phenol-water mixture before you 
is measured while being cooled, there is first observed, at 


THE CHANGES IN STATE OF COLLOIDS 97 


the temperature at which opalescence appears, a sudden 
increase in the viscosity, as shown in Fig. 30. The maxi- 
mum viscosity at this temperature is much above the 
viscosity of either of the two pure components. If the 
mixture is further cooled, the opalescence disappears, 
giving way to a white turbidness characteristic of the 
relatively coarse dispersoids. This is the state that the 
phenol-water mixture has now assumed (demonstration). 
If we measure the viscosity of this white emulsion, we 
find it considerably less than that of the opalescent colloid 
mixture. Paralleling this appearance and loss of opal- 
escence we find the viscosity attaining a maximum in the 
realm of colloid separation, and then falling again as shown 
in Fig. 30. | 

To give you some conception of the quantitative side of 
these changes in viscosity, let me read you a few figures 
that have been observed in the carefully studied critical 
fluid mixtures of isobutyric acid-water. While the viscosity 
of pure water at 20.12° C. has a value of 1.1245, and that 
of pure acid at 20.08° one of 1.983, the viscosity of a critical 
mixture of 59.93 percent acid has a value of 3.677 at 20.99°. 
The changes in viscosity become still more evident when 
the logarithms of viscosity are compared with the tem- 
peratures, as in the accompanying Fig. 30. 

This analogy between the behavior of critical fluid mix- 
tures and that of solvated emulsoids throws light upon 
some of the changes that take place in the process of gela- 
tion. The sudden increase in viscosity of a critical fluid 
mixture when cooled is analogous to the stiffening of a 
solvated colloid and the process of gelation. There occurs 
in gelation, as in critical fluid mixtures, a separation of 
the colloid —a conclusion that may be verified by ultra- 
microscopic and other methods.? 


1 See WotraanG Ostwap, Grundriss der Kolloidchemie, 1. Aufl., 347, 
Dresden, 1909. The ultramicroscopic phenomena predicted here were sub- 
sequently discovered and developed by R. Zstamonpy, W. Minz, W. Bacu- 
MANN and others. See their papers in the Kolloid-Zeitschrift. 


98 COLLOID CHEMISTRY 


§9. 


In the. gelation of colloids, such as gelatin, agar, albumin, 
etc., a separation into two well-marked phases takes place. 
As in phenol-water mixtures, there are formed, first, a con- 
centrated phase rich in colloid and containing little water, 
and second, a dilute phase rich in water and relatively poor 
in colloid. This is true of those colloids at least which in 
the presence of sufficient water always have the tendency 
to pass over into the liquid state and which have never 
been observed in solid, for example, crystalline form. It 
is possible, however, for phases which originally separate 
out as fluid droplets to yield later tiny solid crystals. This 
occurs in the course of time in the gelation of silicic acid 
and certain soap solutions for instance. Both silicic’ acid 
and certain soaps originally give rise to gels which look 
much like the ordinary emulsoid gels, yet they are dis-— 
tinguishable from these by not possessing much elasticity. 
But that which is common to all these gelations is the 
phenomenon of separation; in other words the decrease in 
the degree of dispersion of the whole system on the one 
hand, and the distribution of the dispersion medium in 
unequal concentration in the two phases on the other. 

It might now be urged against this comparison that in 
a critical fluid mixture the separation of the two phases, 
when the dispersed state is reached, is not maintained, but 
that a coarsely dispersed or even non-dispersed mixture 
of the two liquids ultimately comes to pass. But even 
this has found its analogue in jellying colloids. GraHam 
first observed it and his findings have been commented 
upon many times since, though little attention has been 
paid to these comments. 


1 T have during the past years made many experiments upon syneresis, 
without, however, having found time to bring them to a conclusion and to 
publish them. Not only the term itself but all reference to syneresis seems 
to have disappeared from the literature. None of the ordinary text-books 
of colloid chemistry, for example, touch upon it. I hold it and its study 
as extraordinarily important, as may be inferred not only from what is said 
above, but from some paragraphs which are to follow. The phenomena 


THE CHANGES IN STATE OF COLLOIDS 99 


When any gel is left to itself for a number of hours or 
days, protected not only against infection with micro- 
organisms, but also against evaporation, a separation into 
two phases takes place. Every bacteriologist working with 
solid media has observed this. Agar slants, for example, 
squeeze off fluid droplets in the course of time, which 
coalesce to form considerable volumes of liquid. The 
liquid is usually called condensation water, a designation 
liable to confuse one, for the liquid is not the product of 
condensed water vapor, but is actually secreted by the 
colloid. The liquid is, moreover, not pure water but a 
solution of all the constituents of the gel in both colloid and 
molecular degrees of dispersion.' Only these constituents are 
present in a different, that 1s to say lower, concentration. 
The ‘‘serum” given off is therefore in reality a second 
colloid solution secreted by the concentrated colloid, the 
whole being quite analogous to the separation phenomena 
noted in critical fluid mixtures. GRAHAM called this sep- 
aration synerests. It is strange how little this theoreti- 
cally and practically important phenomenon has _ been 
studied. 

I have never seen a gel which does not show syneresis. 
Not only do agar and gelatin (in which the amount of the 
fluid given off increases with decreasing concentration 
of the colloid) show this behavior, but so do starch, silicic 
acid (the concentrated gels of which give off more fluid 
than the dilute), rubber, collodion, cellulose, the gelatinous 
sodium chlorid which I have already shown you, ete. 

I show you here the syneresis of a gelatin, a silicic acid and 
a viscose gel (demonstration). In order to prove that 


of syneresis not only cover, in my mind, a group of changes in state codrdinate 
with the phenomena of swelling and of gelation, but they are possessed of 
possibilities for application to scientific and technical problems of the great- 
est importance. See, in this connection, the last two lectures in this volume. 

*1 According to unpublished experiments, I demonstrated the presence in 
the syneretic serum of the colloid responsible for the gelation in the fluids 
expressed by silicic acid, gelatin, agar, starch, sodium chlorid and polymer- 
ized cinnamic ethyl ester. The degree of dispersion of the colloid in the 
expressed fluid is, naturally, always much greater than in the gel. 


100 COLLOID CHEMISTRY 


the fluids which have been expressed are not simply water 
or salt solutions but contain colloid material as well, I 
shall pour off a part of the ‘‘serum” from each of the two 
flasks. To the liquid from the gelatin flask I add a few 
drops of dilute hydrochloric acid and some tannin. The 
white cloud which develops betrays the presence of gelatin. 
To this second test tube I add some copper sulphate; the 
precipitate of copper silicate 
formed shows that the ‘‘serum”’ 
in this case also contains col- 
loid material. 

The syneresis of a viscose gel 
(see Fig. 31) is particularly 
striking because of its extra- 
ordinary extent. About two 
thirds of the original volume 
of the gel separates itself in 
liquid form from a very solid 
cake. The separation, of 
course, takes time, — in this 
instance several years. 

This phenomenon of syn- 
eresis, therefore, closes the ring 
of analogies between the be- 
havior of critical fluid mixtures 
and that of hydrated colloids. 
Let me add that microscopic 
study of colloids to which dehydrating materials have been 
added supports the assumption that separation occurs 
in ‘‘droplet”’ form in the process of gelation,! and that this 
phenomenon is analogous to that observed when benzol 
separates as a coarse emulsion from water. 





Fic. 31. — Syneresis of a viscose gel. 


1 See especially the studies of O. Btrscui1 and W. B. Harpy discussed 
by WouFrGanG OstwaLp, Grundriss der Kolloidchemie, 1. Aufl., 350, Dresden, 
1909. 


THE CHANGES IN STATE OF COLLOIDS 101 


$10. 


But the gel state may be reached not only by allowing a 
liquid mixture to set, but by allowing a solid to absorb a 
dispersing medium. As you know, a disc of solid gelatin 
goes over into a jelly when it is thrown into water. Differ- 
ently expressed, the gelatin swells. Such swelling may be 
observed in other colloids than gelatin and with dispersion 
media other than water. Thus, rubber swells in benzol, 
collodion in alcohol-ether, etc. Let me illustrate some of 
these swellings and in connection with them call your 
attention to the more important properties of the observed 
colloid changes in state. 

I have here a disc of ordinary carpenter’s glue, the under 
half of which has lain in water over night (demonstration). 
You observe the considerable increase in volume of the 
immersed portion, and, at the same time, an interesting 
optical change. The swollen half is white and turbid while 
the unswollen portion has kept its brown color and relative 
transparency. ‘The immersed portion has yielded a jelly 
with corrugated surface, well-marked elastic properties, ete. 
(demonstration). 

The increase in volume when materials swell can be 
demonstrated even better in the following fashion. I have 
here some very thin and but slightly vulcanized rubber foil 
such as dentists and surgeons use. I have cut from it a 
pantaloon-shaped strip as indicated in A of Fig. 32. One 
of these two divisions we will now permit to swell in order 
that we may compare its change in volume with that of 
the unswollen portion. To do this one-half of the divided 
strip is left hanging outside a test tube while the other half is 
tucked inside. I next carefully fill the test tube with cumol, 
or benzol. Since the swelling, like all other colloid changes, 
takes time, I set the tube aside for a few minutes. 

The production of a gel through swelling necessitates the 
existence of certain physico-chemical relationships between 
the material undergoing swelling and the liquid producing 


102 COLLOID CHEMISTRY 


it. Of the reasons for this we know but little as yet. Gel- 
atin swells in water but not in benzol; vulcanized rubber in 
benzol but not in water. Sometimes a definite temperature 
is necessary in order that swelling may take place. Starch, 
for example, does not swell at room temperatures but does 
. at higher ones; potato starch 
begins to swell between 57° 
and 58°C. This swelling tem- 
perature can be determined 
very accurately! by measuring 
the viscosity of a starch sus- 
pension while it is_ being 
heated. As soon as the pro- 
per temperature is reached, a 
sudden great increase in the 
viscosity is noted which be- 
comes particularly evident if 
the logarithms of the viscosi- 
ties are compared as in Fig. 
33. 3 
It is an interesting fact that 
certain crystals, like those of 
albumin, show well-marked 
swelling. Inorganic solids also 
Fig. 32. — Swelling of soft rubber. swell, as do even metals like 
sodium and potassium in the 
presence of liquid or gaseous ammonia. At low tempera- 
tures all these substances swell considerably without losing 
their general forms. When the ammonia is driven off, the 
pure solids or metals are again obtained. On the other 
~ hand, when the ammonia is allowed to act long enough all 
these substances liquefy, yielding at first a doughy, highly 
viscid mass which, in most cases, then goes over into a 


A B 


1 See WoLFraane OstwaLp, Koll.-Zeitschr., 12, 218 (1913); Kleines Prak- 
tikum der Kolloidchemie, 38, Dresden, 1920. Regarding optical methods 
of determining these swelling points, see M. Samec, Kolloidchem. Beihefte, 
3, 129 (1911). 


THE CHANGES IN STATE OF COLLOIDS 103 


colloid solution. The same thing happens when acacia or 
rubber swells. Under the influence of the material produc- 
ing the swelling and especially at higher temperatures, the 
swelling gradually gives way to colloid solution. In some 
colloids, as in acacia, the temperature necessary for this lies 


Hydration of starch 


Temperature —»> 





Fig. 33. — Changes in viscosity during the hydration of starch. 


so low that at room temperature only solution phenomena 
may be observed. But at 0° C. the particles of acacia show 
the normal phenomena of swelling. 

Swelling can occur not only in liquids but also in vapors. 
But there is a difference, under ordinary experimental con- 
ditions, not only in the rate at which the swelling is brought 

1 Regarding the swelling of metals and their salts in ammonia and its 


probable colloid nature, see Wotraana OstwaLp, Kolloidchem. Beihefte, 
2, 437 (1911) where references will be found to the literature. 


104 COLLOID CHEMISTRY 


about, but in the maximum amounts of liquid that may 
be taken up by the swelling substances in the two media. 
A gelatin disc, for example, takes up less fluid in an atmos- 
phere of water than in water itself. 

While I have been speaking our experiment on the swell- 
ing of rubber. has made progress, and so I now withdraw the 
immersed half from the test tube. As you see (Fig. 32, B), 
the amount of swelling even in these few minutes is enor- 
mous. ‘The immersed half is at least half again as long and 
as broad as the unimmersed. Moreover, the rubber in 
swelling has changed in some of its properties. As I shake 
the strip you catch a sound not unlike that heard when 
writing paper is rattled. Please note the relatively great 
velocity at which these changes in state have taken place. 
It is a matter of enormous interest in certain biological 
problems to which we shall return later. 


$11. 


I should like now to call your attention to another swelling 
phenomenon which illustrates swelling in vapor and which 
also takes place with incredible speed. The experiment 
illustrates the close association between swelling and certain 
kinetic processes. I have here some thin leaves of colored 
gelatin which will recall your childhood days. I take one 
that has been cut and decorated to represent a fish, and lay 
it upon a piece of filter paper. When I breathe upon it you 
observe that the gelatin leaf immediately bends or even 
curls up and jumps upward (demonstration). Perhaps you 
think that the movement is merely the mechanical result of 
my having blown upon the leaf. To meet this criticism I 
have fastened a gelatin leaf into a ring stand. When I 
breathe upon it, the leaf bends away from me (demonstra- 
tion). It remains for a time in the new position and only 
slowly returns to the old. This proves that the movement 
is really brought about through an increase in the volume 
of the gelatin on the side of the leaflet which has been 


THE CHANGES IN STATE OF COLLOIDS 105. 


breathed upon, in other words, through swelling. But the 
swelling soon disappears, for the absorbed water evaporates 
rather quickly from so large a surface. 

This experiment demonstrates vividly the rapidity and 
the enormous value of swelling phenomena. It is for this 
reason that they serve us in the making of certain scientific 
measurements. ‘The principle is used in the well known 
hair hygrometer in which the change in the length of a 
stretched human hair through the humidity of the air 
constitutes the principle employed for measuring the latter. 
Why blond hair works better for these purposes than dark 
hair has not yet been settled scientifically. 


§12. 


Of great interest is the effect of the addition of various 
substances upon swelling. Electrolytes and non-electrolytes 
are known which not only favor but also inhibit swelling. 
Among the most powerful of those which increase the swell- 
ing of such colloids as are capable of absorbing water are 
acids and alkalies. Gelatin and fibrin, for instance, in the 
presence of acids or alkalies will absorb several times as much 
water as when these are absent. Cyanids, iodids, chlorids, 
etc., also favor colloid swelling in certain concentrations, 
while sulphates, citrates, phosphates, alcohol, sugar, etc., 
do the opposite. On the other hand, when salt and acid are 
used in combination, the addition of salt usually brings 
about a decrease in swelling, even though the salt is of a 
kind which alone leads only to increased swelling. 

To demonstrate these effects I have here a series of dishes 
containing water, acid, alkali, potassium iodid, calcium 
chlorid and magnesium sulphate. Into each of them has 
been dropped a gelatin dise of standard weight (demonstra- 
tion). To render the discs readily visible, a trace of colloid 
dye, namely, congo red, has been mixed with the gelatin. 
Had a molecularly dispersed dye been used it would, of 
course, have diffused out of the gelatin while this was 


ti 


106 COLLOID CHEMISTRY 


swelling. The gelatin discs have been in these solutions 
some twenty-four hours and, as you observe, the way in 
which the different substances have favored the swelling is 
as follows: 

acid, alkali, potassium iodid, calcium chlorid, water, magnesium sulphate. 


The disc in the acid solution is already swollen to twice the 
size of that in-the pure water. 

I should like briefly to emphasize that, in the process of 
swelling, considerable heat is set free and that considerable 
quantities of energy suffer conversion. Swelling seeds, for 
example, can lift great weights. In a skull filled with dried 
peas and immersed in water, the swelling peas burst the 
bones of the head apart. The old Egyptians used these 
swelling phenomena for quarrying purposes when they drove 
dry wood wedges into the rocks and then made the wedges 
swell by pouring water upon them. 


$13. 


Were you now to ask me what are the internal changes in 
state which permit a solid body and a liquid or vapor to 
become a jelly, I could not give you a short and simple 
answer. I can only emphasize the following. There is, in 
the first place, a great similarity between a swelling system 
and a syneretic emulsoid system. In the swelling of the 
colloid, as I need to emphasize at this time, a small portion 
of the swelling substance always passes over into a colloid 
solution. In syneresis, on the other hand, we have also to 
do with a concentrated, practically solid phase covered by 
a dilute colloid solution. Swelling, in other words, repre- 
sents the reverse of syneresis. The whole is analogous to 

1 These gelatin discs are prepared by pouring a concentrated gelatin so- 
lution upon a glass plate. The gelatin is allowed to set, after which it is 
cut into squares which are then dried and weighed. See WoLFGANG OsTWALD, 
Kleines Praktikum der Kolloidchemie, 82, Dresden, 1920. For demonstration 
purposes the discs are immersed in %o to 4%o normal hydrochloric acid or 
sodium hydroxid. Half normal potassium iodid, % normal calcium chlorid, 


or highly concentrated (saturated) magnesium sulphate solution may be 
used to demonstrate the salt effects. 


THE CHANGES IN STATE OF COLLOIDS 107 


what may be observed when a gelatin gel, which has been 
kept at a low temperature and which shows the phenomenon 
of syneresis, is again exposed to a higher temperature. In 
both instances a pronounced diphasic, non-dispersed sys- 
tem gives way to a simple dispersed system, namely, the 
gel. In the process of swelling, two macroscopically differ- 
entiated systems give rise to a single dispersed one. 

The power of a solid body to yield a gel through swelling 
seems to be connected with the presence of a definite struc- 
ture. ‘Through the extensive and masterful studies of O. 
ButTscHui, G. QUINCKE and others, it has been proved that 
microscopic and ultramicroscopic discontinuities —in the 
form of meshes, cells, foams, honeycombs, etc. — are widely 
distributed in nature. Not only do those solids which are 
capable of swelling show these structures, but also many 
of the inorganic crystals as those of congealed sulphur 
and the so-called skeletons of potassium permanganate and 
ammonium chlorid. This finding is in harmony with the 
fact cited above that these crystalline substances show a 
behavior closely allied to the phenomena of swelling. That 
metals may show such structure is well known to every 
metallurgist. 

If we believe with O. BUtscuui, G. QUINCKE and others 
that structure is necessary before swelling can occur in a 
solid, then the process of swelling consists, in the first place, 
of an increase in the degree of dispersion of these systems. 
This increase is then analogous to the increase in degree of 
dispersion as observed in the reversion, through heat, of a 
syneresis, or of the effects of age in a colloid. The coarser 
structure of the solid is, as it were, broken up; in other 
words, coarse aggregates are divided into the primary 
particles of which they are composed. As N. GampuKow 
has found, the ultramicroscopic particles of a gel become 
smaller in the process of swelling, or at least lose their highly 
refractive character.! But in the process of swelling there 


1 See N. Gaipuxow, Dunkelfeldbeleuchtung und Ultra-mikroskopie in 
der Biologie und in der Medizin, Jena, 1910; Koll.-Zeitschr., 6, 260 (1910). 


108 COLLOID CHEMISTRY 


occurs another change which may, under certain circum- 
stances, actually run counter to the increase in dispersion. 
The individual particles absorb the medium in which they 
are swelling; they become solvated. This increases the size 
of the particles and so fluid droplets may be formed. The 
two changes, in other words, the combination of increase 
in degree of dispersion with a change in the type of the 
dispersed substance from the side of the solid to that of the 
fluid, seem most characteristic of the process of swelling. 
Accordingly, the process of swelling represents the reverse of 
syneresis. These statements cover what is best established 
at this time regarding the changes of a -disperso-chemical 
nature that occur in the process of swelling. 


§14. 

Let me add a few words regarding the properties of gels. 
Gels show in interesting fashion the properties of both solids 
and liquids. ‘Thus, even when holding 98 percent or more 
of fluid, they may still show maintenance of form and 
elasticity. They may be cut or broken into pieces that hold 
their shapes, and may be bent and regain their original form 
in the same way that solids do (demonstration). On the 
other hand gels show the properties of liquids. Thus, they 
flow when subjected to slow deformation, as when a large 
two percent gelatin cake assumes the shape of the conical 
vessel into which it is introduced. Unfortunately this 
experiment takes too long to permit me to demonstrate it 
to you. I can demonstrate to you this double-sided be- 
havior as determined by the rate at which mechanical def- 
ormation is brought about in a system which, while not 
belonging to the typical gels nevertheless belongs to a closely 
associated group of systems, namely, the doughs. I have in 
this mortar several grams of ordinary potato starch to which 
I shall, while using the pestle, add enough water to make a 
dough. You observe that the material can be poured out 
of the mortar upon a glass plate in a continuous stream 


THE CHANGES IN STATE OF COLLOIDS 109 


(demonstration). There is formed a very large liquid 
“drop.”” When, however, I try to stir this mass with a 
spatula something happens which in no wise agrees with 
the physical constitution of a liquid (demonstration). 
My rapidly moving spatula drives before it lamelle possessed 
of sharp edges, in other words, masses which show all the 
characteristics of solid bodies. If I wait a moment the 
sharp edges disappear, they become rounded and the lam- 
elle again ‘‘run together.”’ I can also take. a large mass of 
this dough and by handling it rapidly knead it into a ball 
(demonstration). But when I place this ball upon a plate 
it liquefies at once under the influence of its own weight. 
If I had kept the ball in my hand the material would have 
flowed through my fingers. You observe then, that de- 
pending merely upon the rate of the mechanical manipu- 
lation a structure may behave at one time like a liquid and 
at another like a solid. 

The free movement which molecularly dispersed particles 
show in gels is another argument in favor of their fluid nature, 
a phenomenon which we observed in our first lecture when 
discussing diffusion in gels. How now can we understand 
this remarkable combination of solid with liquid proper- 
ties? 

Our previous remarks led to the conclusion that gels are 
systems which, on the one hand, are more coarsely dispersed 
than the liquid sols from which they are prepared, while they 
are, on the other hand, more highly dispersed than the solids 
from which they may be prepared through swelling. The 
gels evidently occupy a middle position, so far as degree of 
dispersion is concerned. Furthermore, the degree of dis- 
persion in a gel may vary steadily from values approximat- 
ing those of the coarse dispersions to those of the colloids. 
Through not considering this fact much useless debate has 
arisen. It is, for example, of no purpose to ask whether 
gels have only a microscopic or only an ultramicroscopic 
structure, since both may be found not only in different 
gels, but at one and the same time in any gel. As O. 


110 COLLOID CHEMISTRY 


BtTscH1i1 assumed and as proved in the ultramicroscopic 
investigations of R. Zsiamonpy and W. BacHMANN among 
others many gels have a dual structure. They may con- 
tain ultramicroscopic particles which, in their turn, may coa- 
lesce to give rise to structures of microscopic dimensions. 
This behavior is common and entirely in harmony with the 
theory of gelation sketched above. 

A gel may, moreover, actually have a structure of micro- 
scopic dimensions without this being recognizable by micro- 
scopic or ultramicroscopic means. As I emphasized be- 
fore, a difference in degree of refraction is necessary to 
permit optical differentiation. In highly solvated colloids 
such differences may not appear. Nevertheless, a gel 
remains a gel even though it appears optically homogeneous, 
as in the case of gelatin. Sometimes through treatment 
with alcohol or other dehydrating agents, we can bring about 
refraction differences in such gels, which then make possible 
optical differentiation of structure. By the use of such 
agents we may change quantitatively the characteristic 
properties of a gel, as its maintenance of form, its elasticity, 
its permeability to molecular dispersoids, etc., but we do 
not change its qualitative character. The fact remains that 
the degree of dispersion of a gel is less than that of the liquid 
“solvent’’ and higher than that of the solid from which it 
was derived. Between these limits the degree of dispersion 
may assume any value. When gels are produced, as is 
most commonly the case, by allowing colloid solutions to 
cool, we may say that the characteristic structure of these 
systems depends upon the size of the ‘‘primary”’ colloid 
particles composing it, which means that 2t must at least lie 
near the realm of colloid dimension or actually within that of 
microscopic visability. Whether in such gels the possibility 
always remains of recognizing these ‘‘primary”’ colloid 
particles (as by ultramicroscopic means) is a question of 
secondary importance, which, in different cases and with 
different degrees of dispersion and of solvation, may have 
different answers. hem 


THE CHANGES IN STATE OF COLLOIDS ET 


But just as the degree of dispersion may vary, so also may 
the type of the elements composing the gel. In gels produced 
by swelling, I do not know of an instance in which the dis- 
persed elements are solid or crystalline in character. They 
are, apparently, always liquid. ‘There are, however, cases 
in which an ultimate separation in solid crystalline form 
occurs. Thus, the gelatinous precipitates of metal hydroxids 
may after a time lose their typical gel properties and become 
crystalline. This can also be noticed in silicic acid gels 
which, in the course of time, become brittle and break up 
into inelastic masses. The belief that we deal in this in- 
stance with the formation, initially, of an emulsoid gel 
which changes later to a suspensoid gel is borne out by re- 
cent Rontgenological studies which show that a normal 
silicic acid gel has no crystalline structure while one which 
has been heated, has. A marked elasticity is usually more 
noticeable in emulsoid than in suspensoid gels, even when in 
the latter case the crystalline elements, through union with 
each other and similar phenomena of agglomeration give 
rise to the mechanical circumstances necessary for the at- 
tainment of high elasticity values. 


$15. 


I leave this somewhat theoretical discussion in order to 
acquaint you with some further properties of gels (demon- 
stration). Since gels are permeable to molecular disper- 
soids, chemical reactions may be permitted to occur in gels 
by allowing two molecularly dissolved substances to diffuse 
toward each other in a gel. If I half fill a test tube with a 
gelatin solution containing some potassium bichromate and, 
after the gelatin has set, pour some silver nitrate solution 
upon it, the two dissolved substances diffuse into each other. 
The silver nitrate, particularly, will diffuse downwards into 
the gel, as I have previously demonstrated to you. But 
here the silver salt meets the bichromate and, exactly as in 


1 §. Kyroronuos, Zeitschr. f. anorg. Chem., 99, 197, 249 (1917). 


112 COLLOID CHEMISTRY 


free diffusion, a precipitate or silver chromate is formed, 
the amount of which increases as the diffusion progresses. 
But something happens in this reaction as it occurs within 
a gel which does not take place when diffusion is ‘‘free.’’ 
Were no gel present, the amount 
of the precipitate would increase 
progressively with the course of 
the reaction. But in the presence 
of the gel, when the conditions 
for the experiment are properly 
chosen, there occurs a periodic 
precipitation. In the test tubes 
that I pass around, instead of a 
continuous column of silver chro- 
mate, you observe a series of 
rings or layers of this substance 
between which the gel is practi- 
cally colorless, indicating the ab- 
sence of any precipitated silver 
Fig. 34.— Periodic _ precipi- ake : os fgets ¥ eepeson 
tations of lead chromate ac-  ‘ imilar periodic precipitations 
cording to E. Hatscuex. may be obtained by allowing 
other reactions to occur in gels. 
In Figs. 34 and 35 are shown some beautiful precipitates of 
lead chromate prepared by E. Hatscuex.! 

I can demonstrate this phenomenon of periodic precipita- 
tion still better by utilizing a projection apparatus and some 
plate preparations. In these the gel containing one salt has 
been poured upon a glass plate. After setting, the second 
reacting solution has been dropped upon it in one spot or 
has been painted upon the gel in the form of a ring (Figs. 
36, 37 and 38). You observe in Figs. 36 and 37 how the 
spot made by the original drop is surrounded by a vast 
number of dark rings marking the periodic precipitations of 





1 See E. Hatscuex, Koll.-Zeitschr., 8, 193 (1910); 9, 97 (1910); 10, 77, 
124, 265 (1911); 14, 115 (1914); also the many papers of R. E. Limsreana, 
E. Ktster and others in the Kolloid-Zeitschrift. 


THE CHANGES IN STATE OF COLLOIDS 3 





Fic. 35. — Periodic precipitations of calcium carbonate according 
to E. HaTtscHEK. 





Fig. 36.— Periodic precipitations of silver chromate in gelatin accord- 
ing to R. E. Linsecana. 


114 COLLOID CHEMISTRY 


silver chromate. Fig. 38 shows the results when a ring is 
painted upon the gelatin. 

These periodic precipitations in gels are known as LinsE- 
GANG rings in honor of their discoverer. The theory of their 
origin is still a matter of debate. Even the pretty and long 





fo a ee ive OL EX “ pe ¥ 
Fig. 37. — Silver chromate rings in gelatin. 
accepted explanation of WinHELM OsTWALD seems inade- 
quate in the light of newer studies. Should any of you like 
to make some of these interesting preparations for your- 
selves, let me emphasize that the phenomenon is obtained 
in good form only when definite concentration relationships 
between the materials necessary for the formation of the 
precipitates are maintained.? | 
Fig. 39 illustrates another interesting property of gels. 


1 See especially the more recent publications of E. HatscHEex in the Kol- 
loid-Zeitschrift. 

2 Instructions originating with R. E. Lrsssaana which I have found to 
give splendid results read as follows: a gel is prepared from 4 grams of gel- 
atin, 120 grams of water and 0.12 gram potassium bichromate. - The silver — 
nitrate solution contains 8.5 grams of the solid salt in 100 cc. of water. 


THE CHANGES IN STATE OF COLLOIDS 115 


When gelatin is painted upon a glass plate and is then 
allowed to freeze, the water in the gel crystallizes, under 





Fig. 38. — Silver chromate rings in gelatin. 


proper experimental conditions,! to form the well-known 


_ 1 Following the method of R. E. Lrssecane (Prometheus, 25, 369), glass 
plates are covered with a thin layer of a 2 to 10 percent gelatin solution 
and before these have dried they are exposed to cold. In order to obtain 
the best pictures, it is well to keep the preparations for some time in the 
cold. The ice evaporates after about a day. The gelatin lamelle dried 


116 COLLOID CHEMISTRY 


frost figures. In the process, the water naturally separates 
from the gelatin or pushes it aside. If the cooled plate is 
now thawed, the ice crystals disappear but the gel maintains 
the shape given it by the frost figures. In this way negatives 
of the ice crystals may be obtained which may be lasting 
and which at times are remarkably pretty. The illustration 
shown in Fig. 39 as well as the method of producing such 
frost pictures is the work of R. E. LimsEaane. 

Finally, I wish to show you a third set of preparations 
which will serve to demonstrate to you the tremendous 
energy changes incident to the changes in. state of gels. 
When a gelatin solution is dried upon a glass plate, as in an 
oven at 100° C., the gel contracts. But at the same time it - 
sticks so fast to the glass that large shell-shaped pieces are 
torn off the surface of-the glass as shown in Fig. 40. I 
have learned that such methods in which common glues or 
ising-glass are employed as the active colloids, are used 
technically in the manufacture of certain types of opaque 
window glass. 


$16. 


I need to hasten on to the consideration of some further 
changes in the state of colloid systems. Since the earliest 
days of colloid chemistry, many studies have been made of 
the coagulation of colloids; in other words, of the decreases 
in degree of dispersion which lead to the formation of micro- 
scopic and macroscopic dispersoids. It may be said fairly 
that our so-called theories of the colloid state, as proposed 
from time to time, have all centered about the explanations 
that they have offered of the process of coagulation. There 
have been proposed electrical, chemical and mechanical 
theories of the colloid state by which were usually meant 
corresponding theories of the process of coagulation. Each 
of these attempted to make one of these principles either 


in this fashion then maintain their structure even if after this treatment they 
are brought back to room temperature. . 


Fig. 39.— Artificial frost pictures in gelatin. 


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118 COLLOID CHEMISTRY 


the only factor or at least the most important factor in the 
process of coagulation. 

In considering the forces that are active in the destruction 
of the colloid state through coagulation, I would like to have 
you recall what was said in our first lecture regarding the 
production of colloid systems. We discovered then many 
methods and many types of energy by which we could bring 
about a change in the degree of dispersion. We may, in 





Fig. 40. — Chipping of a glass plate brought about by the drying of gelatin. 


consequence, expect a similarly great number to be active 
in the induction of coagulation. 

We need not, therefore, expect to find of dominant im- 
portance some one method of coagulation, as we thought for- 
merly, but a whole serves of different although coordinated ones. 
As a matter of fact, any attempt to classify the different 
methods of inducing coagulation shows present in all of them 
a mixture of mechanical, electrical and chemical forces, 


THE CHANGES IN STATE OF COLLOIDS 119 


which together lead to the radical decrease in degree of 
dispersion that we term coagulation. 

To enter for a moment into details, it may be said that in 
the coagulation of suspensoids through electrolytes, electrical 
phenomena play a chief role even though not so exclusive a 
one as has been thought.! Disperse particles having op- 
posite charges are particularly liable to precipitate each 
other. Thus the negatively charged metal sols are pre- 
cipitated through very low concentrations of acids, that is, 
by positively charged hydrogen ions. Other cations, like 
those of the neutral salts, act similarly. On the other hand 
a positively charged sol like that of iron hydroxid is precip- 
itated most easily by bases, in other words, by negatively 
charged hydroxylions. In the case of this colloid the anions 
of the neutral salts are particularly effective in producing 
precipitation. The great influence of the electrical factor 
in all these coagulations is strikingly evidenced by the 
importance of the valence of the precipitating ions. The 
coagulating power of different salts for a gold sol increases 
in the order: 


NaCl MgCl AICI; 


while for iron hydroxid it increases in the order: 
NaCl - NaSO. Nas(CsH;0O;). 


Much smaller quantities of the salts yielding ions of high 
valence are necessary, therefore, to bring about a radical 
decrease in degree of dispersion, than of those yielding ions 
of lower value. 

Oppositely charged ASiRGP precipitate each other; thus 
colloid gold precipitates colloid iron hydroxid, and congo 
red precipitates aluminium hydroxid (demonstration). Pre- 
cipitation in these instances is apparently brought about 


1 Valence is therefore not always or alone responsible for the precipitating 
power of an electrolyte; see WoLtraana Ostwa.p, Koll.-Zeitschr., 26, 28, 


69 (1919). 


120 COLLOID CHEMISTRY 


through electrostatic attraction of the particles, neutraliza- 
tion of their charges and coalescence. It is interesting to 
note how very low may be the concentrations of electrolytes 
which suffice to bring about a precipitation of suspensoids. 
Ordinary india ink, for example, may not be diluted with 
tap water, for the salts contained in this water suffice to 
precipitate the ink. 

Coagulation of hydrated emulsoids through electrolytes 
represents a much more complex problem. We expect this 
for we know that in the precipitation of a protein solution 
through neutral salts, two changes must occur side by side. 
There must first occur a dehydration (which in itself leads 
to an increase in degree of dispersion), and then a coincident, — 
or subsequent coalescence of the particles into coarsely 
dispersed aggregates. ‘The importance of the hydration 
element is strikingly evidenced by the fact that in order to 
render precipitation possible relatively large amounts of 
salt are required. Hydrated emulsoids are for this reason 
more stabile than suspensoids, which we found above so very 
sensitive to low concentrations of different salts. The 
dehydrating effects of neutral salts, as well as the dehydrat- 
ing effects of alcohol, etc., are probably not primarily 
electrical in nature, even though the electrical charge! of the 
substances used may not be without influence. The de- 
hydrating agents follow their own physico-chemical laws, 
which, however, are still but little understood. Electrical 
and non-electrical processes are, therefore, associated in 
the precipitation of hydrated emulsoids through neutral 
salts. Perhaps the matter is best illustrated in the studies 
of W. Pau, which show the whole salt, in other words, both 
ions, to play an important part in coagulation. The effects 


1 See particularly the papers of WoLrGanG Pauui and his pupils in the 
Biochemische Zeitschrift, the Kolloid-Zeitschrift and the Kolloid-chemische 
Beihefte. See for example Wo.trcane Pauui, Koll.-Zeitschr., 7, 241 (1910); 
12, 222 (1913); H. Hanpovsxy, Koll.-Zeitschr., 7, 183, 267 (1910) as 
well as the monograph of Pauut, Kolloidchemie der Eiweissk6rper, Dresden, 
1920, 


THE CHANGES IN STATE OF COLLOIDS 121 


of any salt represent the algebraic sum of its constituent 
ions, the effects of which may be additive or antagonistic. 
The non-electrical nature of dehydrating and coagulating 
effects is most clearly to be observed in electrically neutral 
or but weakly charged emulsoids, while in the heavily 
charged, such as acid or alkali proteins, the electrical effects 
again come to the front. In the precipitation of acidified 
(positively charged) proteins, the coagulating power of the 
neutral salts follows the well-known HoFMEISTER series. 
Anions follow the order: 


chlorate, nitrate, chlorid, acetate, sulphate, tartrate; 


the bases, the order: 


magnesium, ammonium, sodium, potassium, lithium. 


These series are reversed when alkalinized protein instead of 
acid protein is used. ‘The important réle of the valence of 
the ions largely disappears as soon as neutral albumin is used, 
though this is still capable of being precipitated by neutral 
salts, according to the experiments of W. Pau. These 
facts prove that in this case the electrical relationships play 
only a secondary role and that this type of coagulation may 
perhaps best be designated as coagulation through withdrawal 
of solvent. 


§17. 


Interesting phenomena are observed when the coagulation 
of mixtures of suspensoids with hydrated emulsoids is 
studied. The greater stability of the emulsoid fraction is 
then carried over to the suspensoid fraction which, in con- 
sequence, becomes less sensitive to concentrations of salt 
which previously coagulated it. We say the emulsoids exer- 
cise a protective action upon the suspensoids and explain the 
phenomenon by assuming that the fluid emulsoid droplets 
surround the suspensoid particles, or in some way combine 
with them — a view well supported, for example, by ultrami- 
croscopic observations. The protective action 1s shown even 
by traces of emulsoids, another fact in harmony with the 


122 COLLOID CHEMISTRY 


explanation just given, for theoretically only very little emul- 
soid material is necessary to surround a suspensoid particle. 

Much use is made in scientific and technical colloid 
chemistry of this protective action of the emulsoids. Thus, 
the presence of traces of gelatin enables us to produce 
more concentrated and more highly dispersed sols than 
can be produced in pure dispersing media alone. The 
effect of tannin in the production of a highly dispersed 
red gold of the type I showed you in my first lecture de- 
pends in good part upon such a protective action. The 
tannin not only acts as a reducing substance, but at the 
same time as a protective colloid. The presence of a pro- 
tecting colloid is also of advantage because it makes pos- 
sible the evaporation of suspensoid sols to dryness. Because 
of the spontaneous solubility of the protective colloid the 
dried material, when thrown into its solvent, can be brought 
back into a state of colloid solution, the suspensoid fraction 
retaining under these circumstances tts original high disper- 
sion value. I show you here a number of such ‘dried 
hydrosols”’ (demonstration). They are, as you see, mostly 
dark colored scales which are readily soluble in water 
(demonstration). 


$18. 


In addition to the methods of coagulation brought about 
through the addition of different substances, others are 
known in which radiant energy, as from radium emanations, 
is active. Coagulation may also be brought about through 
exposure to light, through shaking with charcoal, with 
Fuller’s earth, or with other powders, and through the 
addition of non-miscible liquids. Thus, many colloids, in- 
cluding the proteins, may be separated almost completely 
from their dispersion media by being shaken for long 
periods of time with benzol or petroleum. Moreover, 
when egg white is beaten to a foam, a part is regularly 
coagulated in the walls enclosing the air bubbles. 

A decrease in degree of dispersion to the point of inducing 


THE CHANGES IN STATE OF COLLOIDS 123 


coagulation can also be brought about through centrifug- 
ing, etc. These belong to the mechanical methods of pro- 
ducing coagulation. Certain details of the way in which 
these different methods produce their effects will be dis- 
cussed later. 


§19. 


The reverse of coagulation, namely, peptization, may also 
be brought about by the most diverse chemical, electrical 
and mechanical means. Coagulated gels, for instance, may 
again be brought to a state of colloid dispersion through 
treatment with weak acids or bases, as will be shown in 
the next lecture. Of special importance are the peptization 
phenomena in which an increased dispersion is brought 
about in a gel through the addition of traces of electrolytes. 
Freshly precipitated sulphids may thus be brought back 
into colloid solution by being treated with dilute hydrogen 
sulphid. 

Let me illustrate peptization to you by an experiment 
devised by A. LotrERMosER (demonstration). These five 
flasks all contain the same amount of freshly precipitated 
silver iodid. To one has been added water only. The 
remaining four contain equal amounts of differently con- 
centrated potassium iodid solutions. The concentration of 
the potassium iodid increases in the order in which I have 
arranged these flasks, amounting in the last one to a one- 
fourth molar solution... You observe how in the water 
and in the most concentrated potassium iodid solution the 
precipitate remains coarsely dispersed. The supernatant 
liquid in these is relatively clear. In the remaining flasks 
this liquid is of milky appearance, particularly marked in 
the middle one containing approximately a ;3, molar 
solution. In this concentration the gel has changed to a 
typical colloid solution of silver iodid. 

Such dispersing effects due to low concentrations of 


1 See A. LotrermMoserR, Koll.-Zeitschr., 2, Supplement 1, 4 (1907); 3, 
31 (1908); 6, 78 (1910); also, Zeitschr. f. physikal. Chemie, 62, 359 (1908). 


124 COLLOID CHEMISTRY 


electrolytes are frequently observed, the ions capable of 
producing them being known as stabilizing, or sol-forming 
ones. Their effect is observable only when the gel is freshly 
precipitated and is possessed of a definite ‘‘mechanical”’ 
constitution. By the latter is meant that, in the fresh 
gel, the colloid particles are merely agglutinated and have 
not yet coalesced. Under such circumstances the ions can 
apparently give the ‘‘primary” gel particles an electrical 
charge leading to their electrostatic repulsion which in 
turn leads to the re-solution of the gel. 

In many cases, both in emulsoids and in suspensoids, 
mere restitution of the original conditions suffices to make 
a precipitated colloid go back into solution. We say then 
that the colloid change is reversible. Generally speaking, 
reversible coagulations are commoner among the emulsoids 
than among the suspensoids. Still it is wrong to consider 
irreversibility of precipitation as directly characteristic of 
the coagulation of suspensoids, as is still done. Colloid 
silver, for example, yields a coagulum upon the addition 
of ammonium citrate or ammonium nitrate which is en- 
tirely reversible.1 


§20. 


In concluding this lecture, let us consider briefly that group 
of colloid changes in state which are comprised under the 
heading of adsorption. In keeping with what was said at 
the beginning of this lecture, we may regard adsorption as 
that change vn concentration which colloids and other dispersed 
systems suffer at the surfaces where they come in contact with 
other bodies. This change in concentration is the only 
constantly observed behavior that is common to all the 
myriad manifestations generally grouped under the term 
adsorption. We shall, therefore, consider only this phase 
of the problem. After such a concentration difference has 
come to pass, a long series of secondary changes may take 
place. 


1 See 8S. OpEN and E. Ox Lon, Zeitschr. f. physikal. Chemie, 82, 78 (1913). . 


THE CHANGES IN STATE OF COLLOIDS 125 


In cases of positive adsorption there commonly occurs 
a fixation of the colloid or of the dispersed material upon 
the solid, liquid or gaseous adsorbing surface. Under such 
circumstances, the rest of the dispersed system may be 
poured off without carrying away with it the dispersed 
material that has collected in the surface. Such fixation 
in the surface may be brought about, for instance, by 
the colloid being coagulated into a coherent layer. As a 
matter of fact, the mechanical coagulations discussed above 
rest in part upon such primary adsorption effects. The 
increase in concentration may go so far that the adsorbed 
material separates out in solid or even in crystalline form 
upon the edge of the adsorbent, as observable in the ad- 
sorption of organic dyes by charcoal. At other times the 
adsorbed material may wander into the adsorbent and there 
form a liquid or solid solution. This can occur of course 
only when the adsorbed materials are capable of diffusion, 
in other words, are molecular dispersoids. The phenom- 
enon is illustrated by the adsorption of iodin by charcoal. 
Finally, in consequence of an accumulation of the dis- 
persed particles in a surface, chemical reactions of various 
kinds, more especially polymerizations, may occur. Such 
are actually observed, for example, in the adsorption of 
amylenes.!_ More pronounced chemical reactions like hy- 
drolyses, oxidations, etc., may also be observed. But all 
these changes are secondary and may be of totally differ- 
ent types in the different special cases of adsorption studied. 
The primary and only constant change which character- 
izes adsorption is found in the change in concentration of 
the dispersed material that occurs in the surface layer. 

Before showing some adsorption experiments, it is im- 
portant to emphasize that the intensity of adsorption is 
chiefly dependent upon the size of the adsorbing surface. 
It is for this reason that for practical adsorption purposes 
we use highly dispersed powders of carbon, fuller’s earth, 
etc. The enormity of the adsorbing surfaces in such dis- 


1 See L. Gurwitscu, Koll.-Zeitschr., 11, 17 (1912). 


126 COLLOID CHEMISTRY 


persed systems is not generally appreciated, nor how 
greatly these grow with increase in the division of the 
particles. To illustrate the matter I append the following 
table which shows the surface increase of one cubic centi- 
meter when undergoing decimal division: 


INCREASE IN SURFACE OF A CUBE WHEN D&EcIMALLY DIVIDED. 








Length of Cube Edge. Number of Cubes. Total Surface. 
ASOT) ities, ca akh, BESS Cording sak Ta oetan 1 6 sq. cm 
SEYLER rem ees IO SN Oe UN 108 CO 
Di ieee eee Ok oe, te eee 10° 600 =“ 
O01 trim, sean acamt at eae eater a 10° 6,000 « 
1 ech ERE, Ca Nee Ce aN 10” 6 sq.m 
SRN EB eek SOS Inn rs Ba ne Ae ar 10% GO ee 
OE UD Lr ae ere pee SE Oe DL eR RS 10" 600 ‘ 
Ly es ee Cake te Oe ee Bee 101 26,000. ae 
ad rid sia aa sity oe cee ee ee ae 1074 60,000 ‘ 
OO ies 2 cd rene ee eae eee ig" 600,000 ‘ 
D 2001 aia ac Se Foes os ees See a 10” 6 sq. km 





In this table is shown how a solid one centimeter cube 
of carbon acquires a total surface of 60 square meters when 
divided to the point of microscopic visibility (0.1 »), and one 
of 60 to 600 square meters if the subdivision is carried 
through the colloid realm. A sugar manufacturer. buying 
a cubic meter of charcoal for clarifying purposes receives 
when this consists of particles one millimeter in diameter, 
some 600 square meters of surface; while if the particles 
are 1 » in diameter he receives six million square meters, 
or six square kilometers of adsorbing surface for his money. 
These figures illustrate to what extraordinary values these 
surface increases mount when dealing with highly dispersed 
adsorbents. 

I want now to show you the wide distribution of these 
adsorption phenomena (demonstration). I have here a 
number of flasks filled with differently colored colloid and 
molecularly dispersed liquids: iron chlorid, fuchsin, ber- 
lin blue, congo red, colloid silver, colloid gold and 
colloid graphite. Standing opposite is an equal number 


THE CHANGES IN STATE OF COLLOIDS 127 


of flasks, each containing a spoonful or two of bone char- 
coal (bone black). I pour upon the bone black in each of 
the flasks the contents of the flask opposite it and for a 
moment shake every mixture vigorously. I next pour all 
the mixtures, along with their bone black, one after the 
other into this large filter-lined funnel. In spite of the 
fact that I have carried out this whole procedure as rapidly 
as possible, you see that the filtrate runs through entirely 
clear. Adsorption has, in other words, occurred with 
extraordinary rapidity in all these cases and is practically 
complete. 

To demonstrate that not only colored dispersoids and 
not only charcoal show such prompt adsorptive effects, I 
show you an experiment with a colorless alkaloid, quinin 
bisulphate, and a special preparation of fuller’s earth, pre- 
pared by Joun Uri Luoyp of Cincinnati;! 10 cc. of the 
clear quinin solution are poured upon a half gram of the 
dry adsorbent, the flask is shaken and the mixture allowed 
to stand a moment. To prove to you that the original 
quinin solution was rich in this alkaloid —it was a 2.5 per- 
cent solution —I acidify it slightly and add Mayzr’s 
alkaloid reagent to it (demonstration). A heavy white 
precipitate forms at once. Let me now filter the adsorp- 
tion mixture and repeat this test with the filtrate. One 
hardly expects, of course, that adsorption will be complete 
and that our filtrate will not show some slight turbidness, 
and yet as I perform the test you observe that the filtrate 
remains absolutely clear no matter how much reagent or 


acid I add? 


1 IT am much indebted to Professor J. U. Litoyp for giving me the ma- 
terial necessary for. this demonstration. The preparation appears in the 
trade under the name of Luoyp’s Alkaloidal Reagent. 

2 According to the accompanying circular the following amounts of 
Lioyp’s reagent are required. To adsorb completely one gram of cocain- 
hydrochlorid, there are required of the reagent 10 grams; for one gram of 
strychnin sulphate, 10 grams of the reagent; for one gram cinchonin sul- 
phate, 10 grams; for one gram cinchonidin sulphate, 10 grams; for one 
gram quinin bisulphate, 8 grams; for one gram atropin sulphate, 8 grams; 
for one gram brucin sulphate, 5.6 grams; for one gram codein sulphate, 


128 COLLOID CHEMISTRY 


When specific instances of adsorption are studied we 
observe considerable differences so far as intensity of 
adsorbing power is concerned. Acids, for example, are 
usually better adsorbed than their salts; and organic salts, 
better than inorganic. Substances of high molecular 
weight and colloids are especially well adsorbed. The 
ready adsorption of the latter has been regarded by many 
authors as specific of them. Of course there exist quanti- 
tative differences among these, too. If, in my previous 
experiment, I had chosen to mix a concentrated arsenious 
trisulphid with bone black and had poured this mixture 
upon the filter, I might not have been so successful in get- 
ting an absolutely clear filtrate. 

Of much importance are certain quantitative relations 
as observed when adsorption is allowed to occur from 
solutions of different concentrations. Generally speaking, 
adsorbents take up relatively more from dilute solutions 
than from more concen- 
trated ones. In most 
instances an adsorption 
maximum is attained be- 
yond which no increased 
amounts of the adsorbed 
material are taken up by 
the surface of the adsorb- 
ent. The “concentration 
Fic. 41. — Diagram illustrating the function’? of adsorption 

concentration function in adsorption, when graphically expressed 

in solution and in chemical combi- has, therefore, a hyperbolic 

nation. . ° 

form as shown in Fig. 41, 
A. This concentration function of adsorption differs 
markedly from the concentration function expressing the 





5 grams; for one gram morphin sulphate, 4 grams. The mixtures are usu- 
ally prepared in from 100 to 200 ce. of water. 

* In modern literature we: frequently encounter the term adsorption 
isotherm as a designation for this concentration function. The use of this 
term represents a by no means justified coquetry with the simpler systems 
of physical chemistry containing but three variables (as is the case, for 


THE CHANGES IN STATE OF COLLOIDS 129 


distribution of a dissolved substance between two non- 
miscible liquids. The distribution of a salt, for instance, be- 
tween water and chloroform is represented by a straight line 
as shown by B in Fig. 40. For purposes of comparison, we 
may introduce the effects of the formation of stoichiometrical 
combinations of a chemical nature between adsorbent and 
adsorbed material. The two molecules reacting with each 
other might under these circumstances be regarded as 
adsorbent and material to be adsorbed, the adsorbed 
amount being calculated as number of molecules bound. 
Saturation of all the molecules considered as adsorbent 
by the material to be adsorbed would under such circum- 
stances not be attained until a definite stoichiometrical 
concentration of the latter had been reached. At this 
point the number of the molecules constituting the ad- 
sorbent would be just enough to yield a definite chemical 
compound. But at this concentration all the molecules 
of the material to be adsorbed would have been used up 
and the composition of the reaction product would then 
not change even when an excess of the molecules being 
adsorbed was added. The whole process represented 
graphically would yield therefore a right angled curve, as 
shown in C of Fig. 41. All possible transitional types 
are discoverable between these three curves. Some of 
them are extremely interesting but their detailed consider- 
ation would lead us too far afield. 


$21. 


If it is asked what forces bring about these concentration 
changes in surfaces, it can only be answered that a whole 
series of different kinds of energy plays a role. That which 


example, in gases) in which, through exclusion of the one variable, the func- 
tion of the remaining two is then easily obtainable. But in adsorption 
equilibria this does not hold true, since at constant temperature one may, 
for example, obtain a whole series of adsorption isotherms, dependent upon 
variations in the amount of absorbing material or variations in its specific 
surface. To speak of an ‘‘isotherm” under such circumstances is to invite 
objection. The expression ‘‘concentration function” is much to be preferred. 


130 COLLOID CHEMISTRY 


is common to all the forces active in adsorption is expressed 
in a generalization of WILLARD GiBBs’ theorem, and reads 
as follows: adsorption will take place whenever there exists in 
a surface a difference in energy potential which can be de- 
creased through a change in the concentration of the dispersed 
materials bordering upon this surface.’ 

To get a picture of what this means, imagine a solid 
dipping into a liquid toward which the solid possesses an 
electrical charge. There exists in the surface, in this case, 
an electrical difference in potential. If a dispersed phase 
carrying a charge opposite to that possessed by the solid 
body is present in the liquid, the difference in electrical 
potential can evidently be decreased by having the dis- 
persed particles aggregate in the surface and so partially 
neutralize it. The-consequence would be an electrical 
adsorption effect. 

Next imagine two substances, such as two non-miscible 
liquids, between which there plays the ordinary surface 
tension. If the one liquid is a dispersoid whose. surface 
tension against the second liquid decreases with increase 
in concentration, there will also be a tendency toward 
positive adsorption, the adsorption being this time mechant- 
cal in nature. This type of adsorption consequent upon 
decreases in surface tension was recognized by WILLARD 
Gress and J. J. THomson, and has in recent years been 
the subject of much study. Its importance seems actually 
to have been overestimated, in that many writers have 
regarded it to be the only possible, or at least the only 
universally important, factor in all adsorptions. 

Imagine, finally, that there exists between two phases, 
as between an adsorbent and a dispersoid, a chemical dil- 
ference in potential. A chemical reaction is proceeding 
in the surface which, like most chemical reactions, increases 
in rate with increase in the concentration of the substances 
concerned in the reaction. Under such circumstances there 


1 This extension of Grss’s rule was made in my ‘Grundriss der Kolloid- 
chemie, 1. Aufl., 434, Dresden, 1909. 


THE CHANGES IN STATE OF COLLOIDS 131 


would also appear a tendency of the dispersed phase to 
accumulate in the surface, thus yielding an adsorption in 
which the driving force would be chemical in nature. 

Similar reasoning governs the effects of thermic and 
photic differences in potential in surfaces. A whole series 
of different energies, in other words, plays a réle in adsorp- 
tion, and these not only may be coodrdinated with each 
other, but may actually at times antagonize each other.! 
These facts will suffice to emphasize the importance of 
distinguishing in any individual case between the different 
kinds of energy that may be playing a part in the produc- 
tion of the adsorption. 


§22. 


Permit me, in conclusion, to touch upon the remarkable 
and interesting illustrations of adsorption that are seen in 
the mutual adsorption of two colloids or in the mutual ad- 
sorption of a molecular dispersoid and a colloid. Which 
in either illustration is the adsorbent and which the ad- 
sorbed material? Obviously in mutual adsorptions of 
highly dispersed systems all differences between adsorbent 
and adsorbed material disappear, just as in the union of 
two molecules with each other chemically. But this anal- 
ogy to the ‘‘purely chemical” reaction goes further. In 
the mutual adsorption of two dispersed phases, the first 
change that occurs consists in a decrease in the degree of 
dispersion of the whole system. This is illustrated in the 
mutual precipitation of two oppositely charged colloids. 
But this is like the formation of a precipitate in any chem- 
ical reaction for which there is also necessary as a first 
change the mutual adsorption of two dispersed systems. 
Further, experiment has shown that the precipitation of 
colloids often necessitates the existence of the reacting 


1 Such antagonistic and complex adsorption processes occur, for example, 
when electrical and surface-tension differences appear simultaneously in 
surfaces. See my Grundriss der Kolloidchemie, 1. Aufl., 435, Dresden, 1909. 


132 COLLOID CHEMISTRY 


materials in definite concentrations... The quantitative re- 
lationships obtaining in the reaction mixture may actually 
yield stoichiometrical values.? These facts further demon- 
strate the analogy between adsorption and the chemical 
union of molecules and show how difficult it may be under 
some circumstances to determine whether the formation 
of a precipitate has been occasioned by purely chemical 
means or by those associated with the physical conse- 
quences of adsorption. Finally, if it is recalled that adsorp- 
tion phenomena are known which are dependent upon 
certain chemical relationships, we become aware of the 
bridge which exists between colloid-chemical reactions and 
the purely chemical ones. The relationship between the 
two appears to be so close that its consequences, when 
applied to our present ‘‘chemical” notions, seem nothing 
short of startling. 


§23. 


I cannot say that with these remarks and with these 
experiments on theoretical colloid chemistry, I have con- 
cluded the subject, though necessity compels me to break 
off here. I can only hope to have made you realize the 
wealth of material that this new science possesses. Had 
I had at my disposal twice the time allotted me, I could 
still only have given you a sketch of the field. In the two 
lectures that follow I shall try to give you a glimpse of the 
scientific and technical applications that may be made of 
colloid chemistry. 

1 See especially the papers of J. M. van Bemmeten, W. Bix7z, etc. Ref- 
erences may be found in my Grundriss der Kolloidchemie, 1. Aufl., 404, 


Dresden, 1909. 
2 See, for example, A. Sanin, Koll.-Zeitschr., 13, 305 (1913). 


IV. 


SOME SCIENTIFIC APPLICATIONS OF COL- 
LOID CHEMISTRY. 





FOURTH LECTURE. 


SOME SCIENTIFIC APPLICATIONS OF COL- 
LOID CHEMISTRY. 


WE shall devote these last two lectures to the applications | 
that may be made of colloid chemistry. Applications may 
be made of a science under two headings. One science may 
be applied to a second, for example. Not only is this 
possible but it must be done in certain instances as when the 
principles of physics or of chemistry are applied to biology 
or mineralogy. We cannot, of course, apply haphazard any 
‘given science to some other. While chemistry, for example, 
may be applied to the biological problem of heredity, the 
converse cannot be done, though into the philosophical 
reasons for this we cannot enter here. Second, we may 
apply science to technical, practical and industrial problems. 

Colloid chemistry also finds application in these two ways. 
In fact I cannot begin a discussion of this question without 
declaring that since the birth of the so-called classical physical 
chemistry of the molecular solutions, some thirty-five years ago, 
no branch of physics or chemistry has arisen which can be com- 
pared in umportance, so far as scientific and technical applica- 
tions are concerned, with that of colloid chemistry. I am 
fully aware of the magnitude of this claim, yet, as this and 
the next lecture will show, I stand ready to defend this 
thesis. I know very well, for example, that radio-chemistry, 
which in point of age may be regarded as a sister science to 
colloid chemistry, has yielded results which have modified 
most drastically and broadened in surprising fashion our 
concept of nature; but so far as the applications are con- 
cerned that may be made of it, either in point of number or 

1 The application of one science to another is regulated by the relations 


expressed in the well-known pyramid of the sciences of A. ComTr and 


WILHELM OSTWALD. 
135 


136 COLLOID CHEMISTRY 


in variety, to scientific and technical problems, even radio- 
chemistry cannot compare with colloid chemistry. 


§1. 

If I have previously complained that the wealth of 
phenomena and ideas embraced in pure colloid chemistry 
was too great to permit me to give you a proper conception 
of it, I can only complain more loudly when I am asked to 
outline the applications that may be made of this science. 
I do not believe that anyone would even today essay to read 
all the papers that have been written, for example, on the 
applications of colloid chemistry to biological and medical 
problems. Whole books are required to give you the 
colloid-chemical views that are of such paramount impor- 
tance in even such special and technical fields as those of 
dyeing and tanning. It is no exaggeration to say that no 
week has gone by in the past ten years (and of course does 
not go by today) in which a new body of scientific or 
technical facts is not recognized as essentially colloid- 
chemical in nature, and in which it is not shown how the 
application of colloid chemistry to such phenomena not only 
brings immediate light but promise of even more in the 
future. In such development over-enthusiasm of course 
will sometimes evidence itself and then problems will be 
designated as colloid-chemical which in reality belong in a 
different category of science. But in spite of such occasional 
lapses, everyone who knows the history of applied colloid 
chemistry will agree not only in admitting that the number 
and the variety: of the problems to which colloid chemistry 
is applicable is already amazingly large, but that the ulti- 
mately attainable by such application can at present hardly 
be imagined. Speculation in colloid-chemical futures is 
still entirely safe. | 

In discussing with you the applications of colloid chemistry 
I must, because of limited time, take my choice between 
presenting a few illustrations in some detail, or a larger 
number more superficially. I shall follow the second course 


SCIENTIFIC APPLICATIONS 137 


which, though less satisfactory in some respects, will best 
serve to emphasize for you the varieties of colloid-chemical 
application. 


§2. 


When we take up the scientific applications of colloid 
chemistry, we recognize at once that a great number of such 
may be made within the field of chemistry itself. We need 
but recall our first lecture to recognize how a discussion of 
precipitates and of their properties of passing through filter 
paper at once plunges us into the midst of that branch of 
chemistry in which we received our first instruction, namely, 
analytical chemistry. The rules laid down by analytical 
chemistry with an eye to avoiding the “going through” of 
a precipitate, such as the working with relatively concen- 
trated solutions, the setting aside of the precipitate, moderate 
heating, the addition of salts, etc., are all of them methods, 
as you see, of utilizing the influences of concentration, of 
ageing, of coagulation and of the effects of salts in deter- 
mining the degree of dispersion of colloids and their pre- 
cipitation. 

Another interesting application of colloid to analytical 
chemistry is seen in the methods employed for recognizing 
traces of the noble metals. In discussing the colors of colloids, 
I called your attention to the great intensity of those shown 
by the noble metals when these are colloidally dispersed. 
At times it may even exceed that of the aniline dyes. It is 
natural that this property of the noble metals should have 
been called upon for analytical purposes, and so it does not 
surprise us that one of the oldest and best known methods 
of demonstrating the presence of traces of gold consists in 
reducing this to colloid form. The Cassius purple test for 
gold is a typical illustration of the production of gold in 
colloid form and its subsequent precipitation in the form of 
an adsorption compound through a second colloid. The 
first step in the test is accomplished through the reduction 
of the gold salt by stannous chlorid. In this way colloid 


138 COLLOID CHEMISTRY 


gold and colloid stannous acid are produced, which then 
unite to form the well-known, reddish-violet precipitate. 

We are familiar with still more sensitive colloid-chemical 
methods of demonstrating minute traces of the noble metals 
of which I should like to show you one (demonstration). 
As shown by the far too little known and appreciated in- 
vestigations of J. Donav, dilute solutions of the noble metals 
are reduced in our common borax beads to solid colloid 
solutions. In this way different colors are imparted to the 
otherwise colorless bead, which differ with the character of 
the metal and its degree of dispersion. Let me fill this 
platinum loop with some powdered borax and then heat it 
in a Bunsen flame. When the salt begins to ‘“‘foam,” I 
moisten it carefully with a very dilute solution of gold 
chlorid and then, by further application of heat, melt the 
whole mixture to a bead. As you observe, the bead assumes 
the rose color familiar to you as characteristic of highly 
dispersed gold. Were I to use a more concentrated gold 
chlorid I should obtain a violet or blue bead. These 
correspond, as you know, with less highly dispersed grades 
of reduced gold. Even the red bead will, with prolonged 
heating, usually turn violet, for under such circumstances 
the dispersion value of the gold is progressively decreased 
until coagulation occurs. 

With silver salts one can obtain blue or yellow beads; 
while platinum salts yield violet ones. The great sensitive- 
ness of these color reactions is of much interest. Spectrum 
analysis is, as you know, generally recognized as of extreme 
delicacy, and yet, so far as the demonstration of the presence 
of the noble metals is concerned, these colloid methods may 
not only equal spectro-analytic ones, but in certain cases 
even exceed them in sensitiveness. The smallest amount 
of gold that may be recognized spectroscopically is 7>423to9 
of a milligram. zo9%057 Of a milligram may be recognized 
by colloid-chemical means. 7 7¢3$77 Of a milligram of 
platinum may be recognized spectroscopically, while z57;90 
suffice for its recognition colloid-chemically. Spectrum 


SCIENTIFIC APPLICATIONS 139 


analysis is more sensitive than the colloid-chemical in the 
case of silver, zososa0 Of a milligram oe silver being recog- 
nizable by the first method against 7543357 by the second. 

In the experiments on the production of colloid gold, I 
emphasized the very different substances that may be used 
as reducing agents. Certain of the organic reducing sub- 
stances act peculiarly energetically in this regard. It is for 
this reason that the formation of colloid gold may be used 
for demonstrating the presence of organic reducing materials. 
The so-called humic acids, for example, which give our soils 
their black color, and which are usually found in the soil in 
colloid form, reduce gold solutions to colloid gold in such 
low concentrations that this reaction has long been used to 
prove their presence.? 

The principle is used in similar fashion in LEy’s test for 
the distinction of natural from artificial honey. In this, an 
ammoniacal silver nitrate solution is reduced by the addition 
of a few drops of very dilute honey. The metallic silver 
that is produced assumes the reddish-yellow color of the 
colloid metal, when natural honey is used, while a darker, 
more greenish, precipitate is formed by artificial honey. 
Traces of albumin or of ethereal oils present in the natural 
product are probably responsible for the difference. Their 
‘‘nrotective action” serves to maintain the colloid silver in a 
higher degree of dispersion in the natural product than in 
the artificial one. While I would not recommend such an 
attempt, it should not be a difficult thing for a colloid 
chemist to do away with this difference between the naturally 
and the artificially produced honey by discovering a material 
which when added to the latter would take the place of the 
natural protective colloid. The practical result of such an 
investigation would probably then bring about a reversal 
in the application of Ley’s test, for the manufacturers 
would in this case, as usually, add too much of the 
substance. 


1 See J. Donav, Koll.-Zeitschr., 2, 273 (1908). 
2 See P. ExRENBERG, Koll.-Zeitschr., 5, 30 (1909). 


140 COLLOID CHEMISTRY 


§3. 


Let me touch next upon some applications of colloid 
chemistry to inorganic and photo-chemistry. A much dis- 
cussed and most fascinating problem in this field concerns 
the chemical nature of the substances comprising the 
latent picture. It is a well-known fact that this depends 
upon the presence of certain reduction products of silver 
_haloids; in other words, upon the presence of compounds 
which contain more silver and less halogen than is repre- 
sented, for instance, by the formula AgCl. These reduction 
products of ‘‘photo-haloids” are, moreover, differently 
colored (yellow, red, violet, blue, etc.). Until recently it 
was thought that they all consisted of ‘‘sub-haloids,’”’ com- 
pounds having some such formula as Ag,Cl. The draw- 
backs to this view reside in the fact that such sub-haloids 
have never been isolated; that one is compelled to hold to 
the existence of a whole series of them (as a, 8, 7, 4, etc., 
haloids) having different colors; and that they must be 
assumed to be able to pass easily from one form to another. 
You will perhaps at this point recall the different colors that 
colloid silver assumes and so yourselves reach a conclusion 
which has been well developed by Lipro-CramerR? in his | 
numerous and careful studies of the subject. The photo- 
haloids are not chemical compounds containing silver and 
halogen in stoichiometrical proportions, but represent 
adsorption complexes of colloid silver in different degrees of 
dispersion with normal, non-reduced silver haloids. 

The correctness of this view has been demonstrated by 
the experiments of W. ReInprERs,? who has succeeded in 
producing these photo-haloids synthetically in the form of 
differently colored crystals by allowing different silver 
haloids to crystallize in the presence of differently colored 


1 See the numerous papers of Liprpo-Cramer dealing with the relations 
of colloid chemistry to photography throughout the volumes of the Kolloid- 
Zeitschrift. See also his books; Kolloidchemie und Photographie, Dresden, 
1908, and Kolloides Silber und die Photohaloide, Dresden, 1908. 

2 See W. Reinpers, Koll. Zeitschr., 9, 10 aa whats references to the 
literature may be found. 


SCIENTIFIC APPLICATIONS 141 


colloid silvers. The crystals took up the colloid silver and 
ultimately appeared with its color. This is certainly 
beautiful proof. 

In passing, I should like to point out that numerous other 
colloids, as the organic dyes, may be thus taken up by 
crystals. Gelatin may also be absorbed. These facts 
deserve much consideration, for they show that the process 
of crystallization in the presence of a colloid does not always 
represent a purification of the crystallizing material. Other 
illustrations might be introduced to show how crystals may 
take up colloids as impurities. In fact, such complexes 
have led to erroneous conclusions regarding the existence of 
different chemical compounds, as in the case of the so-called 
chromisomers, when really none such existed.1 

Time permits me only to mention the fact that many 
allegedly chemical compounds have proved to be colloid in 
nature. Many of the hydrates, for example, are now known 
to be colloid or adsorption compounds, as illustrated in the 
different silicic acids which hold water.2 On the other 
hand, compounds like the allegedly different stannic acids 
have turned out to be one and the same substance existent 
in different degrees of dispersion,®? and the so-called ‘‘solu- 
tions” of the alkali metals and of silver in liquid ammonia 
are probably of colloid nature.‘ 


84. 


The applications of colloid chemistry to organic chemistry 
are not only already very extensive, but promise to multiply. 


1 Such a fictitious color isomerism was discovered, for example, by O. 
Hauser, Ber. d. Dtsch. Chem. Ges., 45, 3516 (1912), in the case of potas- 
sium ferrocyanid, in which through the presence of colloid berlin blue a 
fictitious color isomerism was brought about. I believe that many other 
alleged examples of color isomerism depend upon just such colloid-chemical 
phenomena. 

2 See the numerous papers of J. M. vAN BEMMELEN in Die Absorption, 
edited by WoLFcanG OstwaLp, Dresden, 1910. 

3 See W. Meckiensure, Zeitschr. f. anorganische Chemie., 64, 368 
(1909); 74, 207 (1912); a review is found in Koll.-Zeitschr., 11, 202 (1912). 

4 See WoLFGanG OstwalLp, Kolloidchem. Beihefte, 2, 437 (1908). 


142 COLLOID CHEMISTRY 


All those sticky, mucilaginous, resinous, tarry masses which 
refuse to crystallize, and which are the abomination of the 
normal organic chemist; those substances which he care- 
fully sets toward the back of his cupboard and marks “‘not 
fit for further use,” just these are the substances which are 
the delight of the colloid chemist. For in most instances 
these properties are the properties of colloids, more par- 
ticularly of that group of them known as the solvated 
emulsoids. 

Among the solvated colloids appears a class which we have 
not discussed as yet, namely, that of the ¢socolloids.1 Please 
recall that by the term colloid we mean nothing more than 
a dispersoid in which the degree of dispersion has a definite 
value. Now we cannot only conceive of, but we actually 
know of, instances in which the dispersed phase and the 
dispersion medium have the same chemical composition and 
yet the two do not form a homogeneous or molecularly 
dispersed system. Thus, a polymeric compound is very 
frequently not molecularly soluble in its monomeric form, 
and this is true for many pairs of chemical isomers. To 
illustrate the fact in entirely modern fashion, we need but 
consider the artificial synthesis of rubber through polymeriza- 
tion of isopren. Through: prolonged heating, the isopren is 
polymerized to 4 colloid product, which at first dissolves 
colloidally in the monomeric isopren, greatly increasing its 
viscosity. 

Isocolloids may be produced from a single chemical 
element, for, as you know, certain elements may exist in 
different so-called allotropic states. These may then be 
divided colloidally into each other2 ‘You are all well aware 


1 The concept of the isocolloid was first set up and developed by me in 
my Grundriss der Kolloidchemie, 2. Aufl., 128, Dresden, 1911. 

> The objection of many phase rule theorists that because of the laws 
of equilibrium isodispersoids cannot exist, looks to me like an attempt to 
do violence to nature. These things do exist, as plainly evidenced by the 
numerous mixtures which consist of nothing but one element into which 
that same element has been dispersed in allotropic form. See my Grundriss 
der Kolloidchemie, 2. Aufl., 128, Dresden, 1911. To the examples given 
there might be added other metals, as tin and zine. Just because these 


SCIENTIFIC APPLICATIONS 143 


that silver or phosphorus, for example, may appear in a 
whole series of different physical states or allotropic forms. 
In keeping herewith, we know of a whole series of mixtures 
of such allotropic modifications which have been proved, 
or may be proved, to be colloid mixtures. White phos- 
phorus, on exposure to light, gradually changes to the red 
form. When this change is studied ultramicroscopically, 
the allotropic transformation may be observed directly. 
Under the influence of the light, particles of colloid dimen- 
sions are produced, which gradually coalesce to form larger 
ageregates with a net or honeycomb structure.! In silver 
melts at temperatures between 160 and 200° C., we probably 
also deal with colloid emulsoids consisting of two forms of 
silver. This is rendered probable by the fact that the 
viscosity changes observed in this temperature realm are 
identical with those observable in typical colloid mixtures.’ 
To these isodispersoids, more particularly the isocolloids, 
belong also the various resins, many oils and probably the 
majority of those troublesome organic residues which fail to 
crystallize. 

But illuminating results will also follow the application of 
colloid chemistry to other branches of organic chemistry, as 
to that of the dyestuffs. They are certainly to be expected, 
for a large number of the organic dyestuffs form typical 
colloid solutions in water. Since the properties of such 
solutions depend upon the degree of dispersion, it is to be 
expected that changes in this colloid state of the dyestuffs 
cannot fail to be of importance in the dyeing properties of 
the dyes themselves. Colloid chemistry has already been 
asked to shed light upon certain dyestuff problems, where 
attempts to explain the phenomena observed through the 


systems do not fit into theories of equilibrium, they do not therefore dis- 
appear from nature, nor do they lose in this fashion their great scientific 
and practical significance. 

1 See H. Srepentopr, Ber. d. Dtsch. Chem. Ges., 43, 692 (1910). 

2 See WoLFGANG OstwaLp, Grundriss der Kolloidchemie, 2. Aufl., 131, 
Dresden, 1911; or Handbook of Colloid Chemistry, Trans. by Fiscuer, | 
OrsPeR and Berman, Philadelphia, 1915. See also Koll.-Zeitschr., 12, 220 
(1913). 


144 COLLOID CHEMISTRY 


orthodox ones of chemical constitution have given only 
arbitrary explanations or failed entirely. It has been found 
that a large number of the colloid organic dyestuffs behave 
as do the colloid metals. They change their color, for 
instance, with the degree of their dispersion, passing from 
yellow through red to blue just as do colloid gold or silver.! 
Red colloid gold, for example, becomes blue upon the 
addition of an electrolyte but at the same time coarser. 
Perhaps you know that the color changes of indicators, 
like congo red, are also accompanied by variations in degree 
of dispersion. They used to be attributed to purely chem- 
ical changes. When an acid was added to congo red, for 
example, it was held that the insoluble dye-acid was freed. 
That such a purely chemical explanation is not always 
adequate may be demonstrated with an indicator closely 
allied to congo red, namely, congorubin (demonstration). 
Congorubin changes color not only upon the addition of an 
acid or upon the addition of larger amounts of alkali but, 
what is most important, when all manner of neutral salts 
are added, as sodium chlorid or magnesium sulphate (dem- 
onstration). The addition of a neutral salt brings about 
the change in color even in the presence of free alkali, for 
example in 1/10 normal sodium hydroxid. Barium hydroxid 
and sodium hydroxid for example, change congorubin to 
blue at once (demonstration). There occurs in all these 
cases, as the color changes to violet or blue, a decrease in 
the degree of dispersion of the congorubin as may be proved 
by ultra-filtration, by ultra-microscopic observation or, at 
times, direct observation with the naked eye. The forma- 
tion of a free dye-acid is out of the question in the presence 
of so much free alkali. These experiments and others of 
similar type indicate clearly, therefore, that changes in 
the degree of dispersion of the dye are to be held responsible 


1 See Kolloidchem. Beihefte, 2, 409 (1911); Koll.-Zeitschr., 10, 97, 132 
(1912). ; | 


SCIENTIFIC APPLICATIONS 145 


for the changes in color just as when a gold sol changes from 
red to blue. 


85. 


Colloid chemistry finds many applications, too, in the 
realm of its sister science, physical chemistry. ‘There exists 
the closest possible relation between colloid chemistry and 
capillary chemistry. Colloid chemistry is in reality nothing 
but a special division of capillary chemistry, for both deal 
chiefly with systems which consist essentially of surface. 
On the one hand, the phenomena of surface tension, of 
adsorption and of capillary electricity find immediate appli- 
cation to colloid chemistry, while this sheds new light into 
the field of capillary electricity. 

But there also exist relationships between colloid chemis- 
try and more distant realms of physical chemistry. As you 
know, the classical solution laws of van’r Horr and others 
begin to show exceptions when concentrated solutions are 
studied. Now call to mind, in this connection, what has 
previously been emphasized, that these concentrated solu- 
tions often exhibit the earmarks of the colloid state by 
showing the TyNpDALL phenomenon, by becoming viscid, 
etc. It has been suggested recently, especially through the 
work of American investigators, that in these concentrated 
molecular solutions there occurs a fusing of the dissolved 
particles with the solvent —in other words, solvation. It 
is assumed that the ions or molecules of the dissolved sub- 
stance unite, under certain circumstances, with a large 
number, one hundred or more, of the molecules of the dis- 
persion medium. No one seems thus far to have even 


1 For an exhaustive monograph on the changes in color of congorubin 
considered from a colloid-chemical point of view see WOLFGANG OSTWALD, 
Kolloidchem. Beihefte, 10, 179 (1919); also 11, 74 (1919); 12, 92 (1919); 
Koll.-Zeitschr., 24, 67 (1919). See also the several studies of H. Lutrrs, 
R. Hauuer, R. Keer, etc., in the more recent and current volumes of the 
Kolloid-Zeitschrift. 


146 COLLOID CHEMISTRY 


hinted that whenever a thousand molecules unite in this 
fashion, complexes of colloid dimensions must result as a 
matter of necessity. In such solvates the amount of the 
dispersion medium bound by the molecules varies progres- 
sively and cannot therefore be expressed through simple 
stoichiometrical relations. Careful study of the problem 
shows that between the laws governing such solvation, and 
those which govern the formation of adsorption compounds! 
there exists a whole series of analogies. As previously 
noted, solvation is characteristic of a large number of col- 
loids. A particle of albumin or gelatin, for example, readily 
holds a thousand times its own weight of water. 

These considerations must render it apparent that we are 
destined to discover solution laws which will embrace both 
these classes of dispersoids. The coarse colloids (perhaps 
even the coarse dispersions) will occupy one of the extremes 
under these laws, the dilute molecular dispersoids the other. 
At the present moment these laws are known only in their 
beginnings or are still undiscovered,’ but I believe that the 
view here expressed will bear better fruit than the attempt, 
constantly made now, to make adequate the laws governing 
dilute solutions by everchanging additions and corrections. 
Since the behavior of the molecular dispersoids passes 
gradually over into that of the colloid systems, such more 
general laws as are here discussed would naturally embody 
from the beginning the corrections which need constantly 
to be made in laws governing dilute solutions when the 
attempt is made to make these cover the anomalous be- 
havior of the concentrated molecular solutions. 


1 See Koll.-Zeitschr., 9, 189 (1911); see also the paper of WoLFeaNna 
OstwaLp and K. Munpter, Koll.-Zeitschr., 24, 11 (1919) in which is shown 
that the solvation or swelling of solid gels like rubber or gelatin is governed 
quantitatively by the same equation as the “osmotic” absorption of water by 
cane sugar solutions, etc. 

2 See the preceding footnote. 


SCIENTIFIC APPLICATIONS 147 


86. 


It is not a mere accident that the three most modern 
branches of physical chemistry — those of catalysts, of the 
crystalline liquids and of the radio-active substances — show 
interesting relationships to colloid chemistry. So far as 
. catalysis is concerned — the science of the changes in the 
rate of a reaction through the presence of an added sub- 
stance which does not appear in the products of that re- 
action — I pointed out in the second lecture that colloids 
are peculiarly active as catalyzers. But not only do colloids 
themselves bring about such catalytic effects, but other 
materials rich in surface, even though not colloidally dis- 
persed, act in similar fashion. I need but point out the 
contact effects exhibited by platinum black and other 
metallic powders in the production of sulphuric acid; and 
the use of finely powdered metallic hydroxids, etec., in various 
catalyses as developed by J. Saspatier. It has also been 
discovered that the effect of the walls of the containing 
vessels, as in various gas reactions, is in large measure 
dependent upon their roughness. 

If we try to say how the element of surface favors such 
‘‘heterogeneous catalyses,’’ the effects of adsorption at once 
come to mind. Adsorption, as previously emphasized, 
depends not only upon the absolute but upon the specific 
surface (the quotient of surface divided by volume or weight). 
Therefore when the absolute or relative surface in a reaction 
system is increased, it means increased adsorption, and such 
increased adsorption means increase in the concentration of 
the reacting materials. This process may be further aided 
through the local production of heat which so frequently 
accompanies adsorption. The two processes together will 
serve to explain a large part of the increase in reaction rate 
seen in these systems. Secondary chemical reactions may 
also be made responsible for a group of these catalytic 
effects. Such secondary chemical reactions are also rendered 


148 COLLOID CHEMISTRY 


possible through adsorption effects. As previously ex- 
plained, the great or specific adsorption of one of the con- 
stituents of a reaction mixture may change the whole sys- 
tem of chemical equilibrium in that mixture. These facts 
will suffice to show why it is a matter of paramount im- 
portance that the ferments of the living organism are for 
the most part colloid in nature and why a study of their 
reactions from the point of view of colloid chemistry and 
of adsorption catalysis promises so very much. 

A second subject in physical chemistry to which colloid 
chemistry finds application concerns the liquid crystals, 
or, as they are better called, the crystalline liquids, those 
peculiar substances whose refraction behavior relates them 
closely to the solid crystals. Those of you who are in- 
terested in these substances will know that a question 
concerning them has long been discussed, which, in start- 
ling fashion, is practically identical with that which is con- 
stantly raised in the problems of colloid chemistry. ‘The 
question at stake is whether the crystalline liquids, which 
at times are distinctly turbid or refractively colored! 
and often highly viscid, are homogeneous or molecularly 
dispersed systems, or whether they are heterogeneous sys- 
tems of the type of the emulsions. As in the case of the 
colloids, many facts seemed to argue for the first of these 
conceptions and many for the second. But just as with 
the colloids, discussion of this problem has, strictly speak- 
ing, brought no decision either way. The most widely 
accepted theory of the classical physical chemists is that 
of N. Bosr, which holds that swarms of molecules, in 
other words, loose combinations of a number of molecules, 
swim about in the crystalline liquid. But this concept 
of a “swarm of molecules” is evidently nothing more than 
the designation of submolecular or colloid aggregates; and 
even though this word ‘‘colloid’’ has entered into the 
discussion only recently,2?, more and more evidence is ac- 


1 See the second footnote on page 61. 
2 See Koll.-Zeitschr., 8,270 (1911). 


SCIENTIFIC APPLICATIONS 149 


cumulating to indicate that, at least in many instances, 
the crystalline liquids are neal emulsion colloids. 

Besides the fact that these liquids are turbid or opal- 
escent, we need but emphasize their peculiar behavior re- 
garding changes in viscosity when they are chilled. The 
viscosity curve again assumes a form identical with that 
observed when separation phenomena occur in. critical 
fluids, as during the coagulation of albumin by heat, in 
the separation of sulphur melts,” etc. 

That these crystalline liquids have a colloid degree of 
dispersion has also been proved directly, in many instances, 
by means of the ultramicroscope. ‘Further evidence in 
this direction is offered by the changes in optical proper- 
ties which these systems show upon the addition of small 
amounts of chemically indifferent substances. The close re- 
lation between the crystalline liquids and the colloid state 
is most strikingly illustrated perhaps in the behavior of an 
organic sulphonic acid recently studied by H. SAanpevist.* 
This substance in dilute solution in water behaves as a nor- 
mal electrolyte but in more concentrated form the solution 
not only becomes colloid but shows, at the same time, the 
typical properties of a crystalline liquid. This classification 
of the crystalline liquids with the colloid dispersoids does 
not, of course, explain their optical peculiarities, but, by 
following a lead which concerns an almost forgotten micro- 
scopic phenomenon of capillarity, it is possible that light 
may be found. 

Entirely normal, isotropic liquids, like water, show dis- 
tinct polarization phenomena when they are observed in 
a dispersed state, in other words, in droplet form. The 


1 See especially the newer papers of THe SvepBERG, Koll.-Zeitschr., 16, 
103 (1915); 18, 54, 101 (1916); 20, 73; 21, 19 (1917); 22, 68 (1918). 

2 See Koll.-Zeitschr., 12, 213 (1913). 

8 H. Sanpavist, Koll.-Zeitschr., 19, 113 (1916). 

4 See V. von EBNER, Untersuchungen iiber der Anisotropie organisierter 
Substanzen, 2, Leipzig, 1882; O. Biirscui1, Untersuchungen tiber Strukturen, 
31, 35, Leipzig, 1898, where references may be found to earlier observations. 


150 COLLOID CHEMISTRY 


phenomenon has been designated surface polarization and 
has been attributed to the action of surface tension, which, 
in tiny droplets, comes to assume a considerable value. 
The amount of this surface polarization, on the one hand, 
increases with increasing degree of dispersion. On the 
other hand, its type, sign and value must be influenced by 
the chemical nature and perhaps the shape of the mole- 
cules. It may be—I make this suggestion with all re- 
serve — that we will find here a bridge between the surface 
tension phenomena of microscopic droplets and the optical 
properties of systems, which like the colloids, consist almost 
entirely of surface. 

The third modern — perchance most modern — branch 
of physical chemistry, into which colloid chemistry has 
recently penetrated, is that of radio-chemistry. Some years 
ago I suggested that it would be an especially interest- 
ing feat in synthetic colloid chemistry could the radio- 
active elements be obtained in colloid form.! ‘Two years 
ago it was shown that nature has already made this ex- 
periment. Jt was found that a whole series of aqueous solu- 
tions of radioactive substances are colloid in nature.2 ‘These 
solutions show the phenomena of electrophoresis, of co- 
agulation through electrolytes, they do not diffuse or 
dialyze, are easily adsorbed through other colloids, ete. 
Not all radioactive substances, but the majority, are found 
in this colloid state; hence colloid-chemical methods, 
as those of dialysis, absorption, etc., may be utilized to 
accomplish their separation and concentration. This is 
certainly a very startling and interesting application of 
colloid chemistry. 


1 See my Grundriss der Kolloidchemie, 2. Aufl., 121, Dresden, 1911. 

2 See F. Panetu, Koll.-Zeitschr., 13, 1, 297 (1913); T. GopLewskxt, Koll.- 
Zeitschr., 14, 229 (1914) as well as the excellent review of this subject by F. 
SEKERA, Koll.-Zeitschr., 27, 145 (1920). 


SCIENTIFIC APPLICATIONS 151 


§7. 


But observations upon dispersed (more particularly 
colloidally dispersed) systems have yielded valuable fruit 
in another field which interests chemists and physicists 
alike. I refer to the experimental determination of Avo- 
GADRO’s constant, the famous value N, which states how 
many molecules are contained in a gram-molecule of any 
substance. There exist different methods by which this 
fundamental figure may be determined, of which I shall 
mention only the following. 

As you know, the atmosphere surrounding the earth be- 
comes rarer as we ascend. ‘The matter is expressed in the 
law that with arithmetic progression upwards, the density 
of the atmosphere decreases geometrically. In the kinetic 
theory of gases, this law may be used for determining the 
value of AvoGapro’s constant in that the degree of baro- 
metric change which follows the unit decrease in density 
is inversely proportional to the gram-molecule of the gas.! 
According to J. PmRRIN, this same change im concentration 
under the influence of gravity 1s also shown by dispersoids, 
provided the particles are so small that they show BROWNIAN 
movement. The concentration of the dispersed substance 
_ in any mass of colloid material is, therefore, always greater 
at the bottom of a vessel than at its top. At a given 
height this difference is the greater, the coarser the dis- 
persion. In suspensions of gutta-percha or mastic, in 
which the particles are about 0.3 uw in diameter, the con- 
- centration of the particles 50 » above the bottom of the 
dish is only half that of the bottom, while in the case of 
the earth’s atmosphere, the density does not fall to half 
that obtaining at the surface of the earth until a height of 
six kilometers is reached. But PrErrin was able to show 
that the same law not only holds for the distribution of 


1 For details see J. Perrin, Die Atome, Deutsch von. A. LoTTERMOSER, 
2. Aufl. Dresden, 1920. 


152 COLLOID CHEMISTRY 


gases and of the particles in coarser dispersoids, but that 
one can calculate the value of AvoGApRo’s constant, when 
it is assumed that every dispersed particle behaves like a 
molecule. A ‘‘gram-molecule” of the dispersed particles 
would, therefore, equal their weight times N. The values 
thus obtained are in striking accord with those found by 
other methods and yield a value of 6 to 7.1023 molecules 
in the gram-molecule. 

I can only touch upon the fact that AvoGapRo’s constant 
can also be calculated from the velocity of Brownian move- 
ment and that when this is done the same value is obtained. 
It is certainly remarkable that from the observation of a 
single particle of oil or mercury — even from the study 
of a drop of diluted milk (DreckHuyzeN)—so funda- 
mental a value as that of AvoGapRo’s constant may be 
calculated. 


88. 


I beg you now to follow me to still greater heights. You 
may, perhaps, think that I am joking when I say that 
colloid chemistry has already found interesting applica- 
tions in the realm of cosmic physics and that in the future 
it will find still greater ones. Consideration of our uni- — 
verse will at once reveal to you that in it we deal not only 
with bodies of great mass and with those of molecular 
dimensions, but that it also betrays the presence of dis- 
persoids possessed of very different degrees of sub- 
division. ; 

Of special interest are the dispersoids of the sky as ob- 
served in atmospheric dust and in the atmospheric water 
(steam, clouds, fog, rain and snow). I have already em- 
phasized that the blue and yellowish-red colors of the 
heavens depend upon the dispersoid nature of the atmos- 
phere, and that these color effects rest upon the same 
grounds as the opalescence of typical colloids. In both 
instances we deal with a selective diffraction by particles 


SCIENTIFIC APPLICATIONS 153 


of a diameter less than the length of a light wave. The 
analogy between the opalescence of the heavens and that 
of a mastic solution is so great that the same formula 
governs both phenomena. Even the polarization effects 
observable in them are entirely similar in the two 
cases.! 

I should like, in passing, to direct attention to yet an- 
other optical effect, in which the presence of tiny, light 
diffracting particles plays a great réle, namely, that of 
ordinary daylight. If the light of the sun were not diffused 
through the dispersoids of the earth’s atmosphere, there 
would be no daylight in the ordinary sense of the term. 
The sun, like the moon, would stand in the heavens as a 
_ bright, burning disc upon an entirely black background. 
Wherever the sunlight did not strike directly, there would 
exist deep shadow; there would exist everywhere a garish 
contrast between the lighted and the unlighted. . In short, 
the world would look entirely different. We are indebted 
to the atmospheric dispersoids for our ordinary ‘‘day- 
light.” 

But clouds (which are, for the most part, dispersoids 
of the composition gas + liquid) also behave like colloid 
systems. It is difficult to determine accurately the size 
of the water droplets composing clouds, but we seem to 
deal with particles of approximately colloid dimensions. 
This is proved not only by the fact that they float in the 
air, but also by the degree of polarization shown by the 
light emanating from them. ‘These heavenly dispersoids 
also show a ‘“‘coagulation”’ which is typical of emulsoid 
systems. The product of this coagulation we call rain. 
All this is also not a joke. We even know which factors 
are chiefly concerned in producing the coagulation of these 
heavenly dispersoids. Electrical changes are most im- 
portant in that these bring about a coalescence of the highly 


1 See J. M. Perntrer, Denkschr. Ak. d. Wiss. Wien, 73, 301 (1901); as 
well as his Physikalische Meteorologie. 


154 COLLOID CHEMISTRY 


dispersed water particles and so lead to their ‘‘precipi- 
tation”’ in the true sense of this term.! 

But the applications of colloid chemistry may mount still 
higher. I suspect that a large number of you have read 
SVANTE ARRHENIUS’ interesting volume ‘‘Das Werden der 
Welten.”’ If you have, you will recall that in the theories 
of this investigator regarding the origin of the earth, two 
factors play an especially great réle, namely, light pressure 
and the presence of the universally distributed cosmic 
dust. In the movement of cosmic dust through light 
pressure ARRHENIUS sees one of the most important reasons 
for different cosmic phenomena. In the production of a 
new heavenly body a shifting and an accumulation of 
cosmic dust brought about through light pressure is as- 
sumed to play an important role. It is therefore of great 
interest that K. ScHwaARzSCHILD and certain other physi- 
cists have calculated that the size of these cosmic particles 
is not without influence upon their velocity. Particles too 
small or too large are moved less easily than those of me- 
dium size and calculation yields an optimum for -move- 
ment when they have a diameter of about 0.16 u. But, as 
you will recall, this value places them in the realm of the 
colloids. The size of cosmic dust is therefore ideal for 
these cosmic displacements. This conclusion regarding the 
optical motility of particles of medium size under the in- 
fluence of light pressure has recently been verified experi- 
mentally by F. Exnrenuartr. Colloid particles of a me- 
dium degree of dispersion show an optimal ‘‘ photophoretic” 
velocity. The optimal radius of silver particles moving 
in the concentrated light cone from an are lamp was found 
to be 0.09 to 0.098 u, the absolute velocity being 180 u/ 


1 After WoLFGANG OstwaLp [Koll.-Zeitschr., 1, 333 (1907) and page 118 
of the first (1915) German edition of this book] and P. PawLow [Koll.-Zeitschr., 
8, 18 (1911)] had called attention to the relationship existent between these 
‘‘ dispersoids of the heavens” and the dispersoids of the laboratory, A. ScHMAuUSS 
[Zeitschr. f. angew. Chem., 32, 811 (1919); Chem. Ztg., 884 (1919), etc.] 
took up the problem anew and developed it. 


SCIENTIFIC APPLICATIONS 15 


seconds.!. The illustration again shows the relation be- 
tween degree of dispersion and properties which attain a 
maximum in the colloid realm. That colloid chemistry 
should in this fashion be of importance in the production 
of new worlds, — more than this could hardly be asked of it. 


$9. 


We shall now leave these ethereal regions and glance 
in the opposite direction. Mineralogy, geology, soil chem- 
istry, agricultural chemistry — these are the sciences in 
which colloid chemistry has found brilliant application. 
In fact, it has long been at home in these fields. 

Under the heading of mineralogy we naturally deal chiefly 
with solid colloids. In order to give you at once a par- 
ticularly pretty example, I present these specimens of blue 
rock salt. It was long debated to what this blue color is 
due, for chemical analysis revealed no constant differ- 
ences between the ordinary colorless rock salt and this 
blue product. Organic impurities were often imagined to 
be responsible, while other authors held that, as in the 
“silver haloids,’’ blue-colored sub-haloids of sodium were 
responsible. But recent experiments — more particularly 
ultramicroscopic and synthetic studies— have demon- 
strated that we have to deal with a colloid subdivision of 
metallic sodium in the solid NaCl. Ultramicroscopic 
examination shows blue rock salt to contain numerous 
intensely colored and fairly regularly arranged colloid par- 
ticles which do not appear in the colorless mineral. 

The blue rock salt can, moreover, be produced artifi- 
clally, according to the experiments of H. SIEDENTOPF, in 
the following manner. A piece of colorless rock salt and 
a piece of metallic sodium are together sealed in a glass — 
tube; the tube is evacuated, and its contents then heated 


1 ¥, ExReENHArFT, Physik. Zeitschr., 18, 368 (1917). 


156 COLLOID CHEMISTRY 


to above the vaporization point of sodium. The rock 
salt now assumes a yellowish color which ultramicroscopic 
investigation shows to be due to a molecularly dispersed 
solution of the sodium metal in the rock salt. The yellow- 
colored preparation is then carefully heated a second time 
to definite temperatures, recooled and perhaps heated © 
again. In this way there is obtained, first, a reddish- 
violet, and then a blue rock salt. This treatment of the 
yellow preparation brings about a condensation of the 
originally molecularly dispersed metal into larger particles 
which finally assume colloid dimensions. We deal, in 
other words, with a typical colloid-chemical condensation 
method. 

Let me emphasize in passing that similar procedures — 
the production of a highly dispersed solution which through 
its cooling, reheating and subsequent recooling is made 
to yield a dispersoid of colloid dimensions — are used to 
produce gold ruby glass, ultramarine, certain forms of steel 
and many of the organic sulphur dyes. I must also men- 
tion that the blue color of rock salt is identical with the 
blue obtained when sodium is dispersed electrically in 
organic solvents, or when it is dispersed through the 
effects of different rays like the emanations of radium. 
The appearance of these blue preparations in nature 1s, per- 
haps, due to just such emanations originating in the slightly 
radioactive potassium salts which accompany rock salt. 

It is probable that the colors of many other minerals, 
as those of certain precious stones; are dependent upon 
the presence of colloidally dispersed materials, like the 
colloid hydroxids.? 

We are acquainted with a large number of other phe- 
nomena in mineralogy in which the colloid nature of the 
observed changes is only just becoming known. It has 
actually turned out that the different degrees of dispersion 


* See Wotreane OstwaLp, Kolloidchem. Beihefte, 2, 438 (1911). 
2 See especially C. DorLtrer, Das Radium und die Farben, Dresden, 
10. : 


SCIENTIFIC APPLICATIONS 15/7 


observed in different minerals may be utilized in classify- 
ing them. It was first pointed out by the lamented Aus- 
trian mineralogist, F. Cornu, that under the old heading 
of the “hyaline” minerals we deal with ‘‘mineral gels.” ! 
The existence of minerals in these highly dispersed forms 
is so common that Cornu was led to the formulation of 
his theorem of the isochemites, which states that there exists 
for every crystallized mineral a highly dispersed and there- 
fore colloid double. Thus we know silicic acid not only 
in its crystalline form as quartz, but also in a gel state as 
opal. The latter, aside from its water content and the 
presence of certain impurities, is identical in composition 
with the former. For the hydrated crystallized iron oxid 
or brown iron ore we have a double in the so-called stilpno- 
siderite or yellow ochre; for the anhydrous crystallized 
red iron ore, a parallel in red ochre; for the crystallized 
sulphids of the heavy metal we have doubles in the highly 
dispersed ‘“‘blacks” and “indigos’? (iron black, copper 
indigo, etc.). 

Dispersoid chemistry can also teach us much of use in 
the classification of the different minerals. I show you 
here a series of minerals all of which are composed of silicic 
acid alone or of this plus water (demonstration). To start 
with, you observe the well-known, large quartz crystals, 
following which come progressively smaller ones. Next 
stands the so-called chalcedony which no longer appears 
crystalline even under the microscope. Then comes cacho- 
long, for which the same is true. Here I show you the 
completely amorphous, glass-like, hyalith, which already 
contains several percent of water. Next comes the so- 
called siliceous sinter (fiorite, geyserite) and then the opal 
which in its “‘soft”’ form contains thirty to forty percent 
of water. As the last member in the series, I show you a 
normal silicic acid gel as prepared here in the laboratory. 


1 Nearly all the numerous papers of F. Cornu and his collaborators on 
the relation of colloid chemistry to mineralogy may be found in the Kolloid- 
Zeitschrift from the fourth volume (1909) on. . , 


158 COLLOID CHEMISTRY 


I have, as you observe, placed a series of silicic acid 
minerals before you possessed of widely differing degrees 
of dispersion, beginning as they do with macroscopic crys- 
tals and terminating with a typical colloid. But this is 
just such a series of dispersoids as I have previously shown 
you in the case of sulphur and of sodium chlorid. But what 
ws most important, — the properties of these minerals from 
the quartz to the silicic acid gel change progressively as the 
degrees of dispersion change. 

We have long been familiar with the fact that there exist 
minerals of which we cannot say definitely whether they 
belong in such a group as that of the crystalline quartzes or to 
the microcrystalline chalcedonies. Even chemical methods, 
as solubility in potassium hydrate, do not serve to distinguish 
them sharply from each other. The colloid chemist is able 
to show why these analytical methods must fail and why 
these transition forms which at first sight prove so annoying 
to the systematist are bound to appear. Our second lecture 
showed the solubility of silicic acid in alkalies to vary with 
its degree of dispersion, and progressively with this. In 
the mineral series which I have just shown you, we would, 
therefore, expect the solubility in alkalies to increase steadily 
from the crystallized quartz to the opal and the colloid silicic 
acid. The experiments of W. Micuartis on the solubility 
of quartz in calcium hydroxid support this conclusion. 
Under otherwise constant conditions, he observed the solu- 
bility of a smoothly polished quartz crystal to be about 
zoos Percent; that of a ground crystal, 325°; percent; that 
of melted quartz glass, ;4$> percent. Finely divided but 
still microscopically visible particles of quartz powder 
allowed 12.4 percent to go into solution; while from a highly 
dispersed quartz powder (in which the individual particles 
were less than 1 ») practically any amount could be brought 
into solution and made to go over into chemical combination.! 

Similar generalizations hold for the water content of the 
quartzes. This also increases progressively from chalced- 

1 W. Micwaetis, Koll.-Zeitschr., 5, 9 (1909). 


SCIENTIFIC APPLICATIONS 159 


ony to opal. Colloid-chemically it follows as a matter of 
course that the water content must increase with every 
increase in degree of dispersion; on the other hand, it is to 
be expected that the absolute amount held may vary greatly, 
for every change in the state of the colloid (as induced, for 
example, through admixture with impurities) must influence 
secondarily the water absorption. 

Through dispersoid chemistry we may, therefore, gather to- 
gether certain mineral groups into ‘‘dispersoid families,” the 
‘individual members of which, so far as their physico-chemical 
properties are concerned, pass gradually into each other. 


§10. 


Let me add another interesting example of the application 
of colloid chemistry to a mineralogical problem. Yesterday 
I showed you a periodic formation of precipitates in colloids, 
the so-called LizsreGane rings. Many of you no doubt at 
the time recognized their similarity to the well-known bands 
and stripes which we see in such beautiful form in agates, 
banded jaspers, etc. Such bands are also seen at times in 
certain ores, like gold ores. This similarity between the 
two structures is more than a merely superficial one. Care- 
ful investigations of recent date, many of them the work of 
R. E. Lizsecane himself, have shown that the laboratory 
preparations are not only identical in appearance with the 
corresponding minerals, but their mode of production is 
probably the same. We deal, in other words, in these 
geologic or mineralogical processes with the diffusion of 
molecularly dissolved substances into mineral gels, more 
particularly into silicic acid or silicic acid gels through which 
a periodic precipitation of some second substance dissolved 
in the gel is brought about. In other instances, as when 
the agate formation occurs about a central nucleus, the 
diffusion and the periodic precipitations may occur centrif- 
ugally. Details regarding the whole process may be found 
in R. E. Lizsecane’s volume. 

1 R. E. Lizsecana, Geologische Diffusionen, Dresden und Leipzig, 1913. 


160 COLLOID CHEMISTRY 


In order to show you how exactly these agate formations 
may be imitated in the laboratory, I have prepared the 
following experiment (demonstration). You will recall the 
periodic precipitations of silver chromate which are obtained 
when silver nitrate is permitted to diffuse into a gelatin gel 
containing potassium bichromate. If, instead of allowing 
the diffusion to occur in one or two directions only (as into a 
test tube or from a point on a gelatin-covered plate) the 
diffusion is permitted to occur in three directions, what I 
am going to show you now results. A rather large amount 
(say 500 ec.) of a potassium bichromate gelatin gel is pre- 
pared in a beaker and, after the whole is set, the solid mass 
is carefully taken out of the beaker’ and dropped into a 
somewhat larger one containing silver nitrate. Silver nitrate 
surrounds the gelatinous mass on all sides, and therefore — 
diffuses concentrically into it. After about twenty-four 
hours the silver nitrate solution is poured off, the gelatin 
block rinsed in water and placed upon a dish where it may 
be sliced open with a large, sharp knife (demonstration). 
If the experiment has gone well — this is always nervous 
work, since we cannot look into the middle of the gelatin 
during the experiment — the gelatin is seen to be streaked 
with numerous concentrically arranged bands, which yield 
different pictures, but all of which look strikingly like 
different agates. 

I should like to add that we observe this type of periodic 
structure in many animals and plants and that a similar 
explanation may be given of their origin. Let me direct 
your attention to the volume of E. Ktstmr, which deals with 
the biologic applications of these periodic precipitations. 
Though I would not be understood as maintaining that we 
can at once explain the markings of a zebra or a tiger in the 
terms of colloid chemistry, still there is no doubt that impor- 

1 The gel is best removed by dipping the beaker for a few moments into 
boiling water. If the instructions given on page 114 are followed, it is best 
to carry out the experiment in a refrigerator in order to obtain a thoroughly 


solid gel. It is necessary to use a good quality of “hard” gelatin mae as 
is used in bacteriology. see 


SCIENTIFIC APPLICATIONS 161 


tant and extensive analogies do obtain between colloid 
chemistry and biological phenomena. 


811. 


If we turn to geology, the important effects of weathering 
immediately give rise to colloid-chemical thoughts. We 
are here again indebted to F. Cornu for pointing out that 
the weathering of crystallized minerals nearly always yields 
gels or mixtures of gels. From feldspar we obtain the highly 
dispersed kaolin; from serpentine, talc; from. brown iron 
ore, the yellow ochre, to which the yellow color of clay and 
earth is due. 

A particularly striking example of the by-effects of colloid- 
chemical factors is seen in the formation of deltas. Delta 
formation depends upon the coagulation of grossly dispersed 
and colloid materials contained in the sweet waters of rivers 
by the electrolytes of sea water. Obviously, this coagula- 
tion will occur the more rapidly and be the more intense the 
more concentrated the sea water which meets the river 
water. It is for this reason that the unusually high salt 
content of the Mediterranean has yielded the most famous 
example of delta formation, namely, that of the Nile. 


$12. 


Soil chemistry has also to do with many different disper- 
soids, of which those that are highly dispersed — more 
particularly colloidally dispersed — are especially important. 
What are known as mechanical methods of soil analysis 
are nothing but methods of dispersoid analysis — coarsely 
dispersed particles are separated from more finely divided 
ones by sieving, by sedimentation and by filtration. The 
colloidally dispersed phases are then separated from each 
other by dialysis; the molecularly dispersed, by processes 
of diffusion. 

Of the typical colloids or their gels which we find in soils, 
four kinds deserve particular mention, namely, silicic acid 
and the silicates, aluminium hydroxid and its compounds 


162 COLLOID CHEMISTRY 


with silicic acid (in other words, the clays, etc.), iron hy- 
droxid, and those substances rich in carbon and of unknown 
chemical composition, summed up under the term of the 
humus acids, and a part of which at least are undoubtedly 
colloid. To this list must be added the micro-organisms 
of various kinds — like the soil bacteria — of which many . 
are so small that suspensions of them show coagulation 
phenomena.? We must also add the mucinous substances 
which are secreted by such soil organisms. 

For determining the colloid content of different soils, use 
has recently been made not only of dialytic procedures but 
of the adsorption of dyes like malachite green. 

The important role of the colloids in the soil has been 
more and more emphasized during the past few years — it 
has in fact been maintained by some that ‘‘the fertility of 
the soil is proportional to its colloid content.” This is 
certainly carrying it too far, as best shown by the fact that 
a whole series of methods for improving the soil consists in 
producing a coagulation of the soil colloids. The good 
effects of frost upon a soil are probably due to such a coagu- 
lation. Laboratory experiments show that during a frost 


1 For a discussion of the colloid or non-colloid nature of the humus acids 
—a discussion not yet ended — see the extensive review of H. Bren, 
Koll.-Zeitschr., 18, 19 (1913). According to 8. Optn [Arkiv. f. Kemi. usw., 
.6, Nr. 26 (1912); Koll.-Zeitschr., 14, 123 (1914); Kolloidchem. Beihefte, 11, 
76 (1919) where references to the literature may be found] the humus acids or 
alkali humates obtained in the usual fashion from peat are non-colloid for they 
dialyze, show no ultramicroscopic structure, are not precipitated by salts, 
are adsorbed with difficulty, ete. These same substances separated in sim- 
ilar fashion from loam by Professor SuzuK1, working in my laboratory, be- 
haved in typical colloid fashion. They dialyzed but little, showed a distinct 
ultramicroscopic structure, were easily precipitated by sodium chlorid, were 
readily adsorbed by bone black, etc. These facts corroborate the general 
experience of chemists that the humus substances may appear in all degrees 
of dispersion and that the much discussed question of whether they are 
“colloid” or ‘molecular’? cannot be answered by yes or no. This question 
carries a different answer under different circumstances. 

2 According to E. Hincarp, A. ATTERBERG, etc., quartz suspensions 
begin to show coagulation phenomena when their particles attain a size of 
20 to 200 uw. In this connection, and for a general discussion of the relation 
between colloid chemistry and agricultural chemistry, see P. EHRENBERG, 
Koll.-Zeitschr., 3, 193 (1908); 4, 76 (1909); 5, 100 (1909). 


SCIENTIFIC APPLICATIONS 163 


gels are formed which decrease a soil’s ‘“‘richness.’’ We may 
explain similarly the good effects of “burning” a soil, a 
practice much followed formerly. Under this heading is 
also to be put the application to the soil of such strongly 
coagulating salts as calcium sulphate. All these methods 
not only bring about a coarsening of the soil colloids, but 
they reduce their high indices of swelling which constitute 
the characteristic element of excessively ‘‘rich”’ soils. These 
facts should suffice to show that too large a colloid content 
does not represent the optimum for plant growth. 

On the other hand, it cannot be doubted that the colloids 

are not only important, but that they are absolutely essen- 
tial to the fertility of soil. This was well known even to 
the old agricultural chemists and is proved directly by the 
knowledge that sandy or gravelly soils — be their chemical 
composition what you will — are unfertile. A whole series 
of facts serves to emphasize the importance of a medium 
content of colloid materials. The water content of soil 
must obviously be largely regulated through the presence 
of hydratable colloids. Sandy or other coarsely dispersed 
soils do not hold rain; neither do they draw up water from 
the depths as readily as do soils containing more colloid 
material. The soil colloids, by holding the water which 
falls upon them and by bringing it up from the depths, 
fulfill one of.the most important conditions necessary for 
the growth of plants upon the surface of our earth. 
- But the adsorption power of the soil colloids for dissolved 
substances is also of tremendous importance. Agricultural 
chemistry recognizes this in two directions. There is, first 
of all, the adsorption of nutritive substances necessary for 
the growth of the plants; on the other hand, there is the 
adsorption of materials which are poisonous to plants, or 
which are the product of their metabolic processes. Water 
plants do better, for example, when any highly dispersed 
powder or colloid, such as carbon, iron hydroxid or silicic 
acid, is added to the water. 

The adsorptive activity of the soil colloids so far as nutri- 


164 COLLOID CHEMISTRY 


tive substances is concerned may be either useful or per- 
nicious, depending upon the concentration of the substances 
present and the intensity of the adsorption — the latter 
increasing, other things being equal, with the increase in 
colloid content. This adsorptive activity has a favorable 
action when the nutritive substances concerned are present 
in relatively low concentrations. They are then gathered 
together by the soil colloids and brought to the plant in 
greater amounts. On the other hand, it may be followed 
by evil consequences, as when the concentration of the 
nutritive substances thus brought about exceeds an optimum 
— an effect particularly likely to be produced in the case of 
the salts — or when the adsorptive force is so great that the 
nutritive substances are held too firmly by the soil colloids 
so that the plant roots can no longer take them over in 
optimum amounts, or at an optimum rate. The good 
‘effects of using calcium salts after fertilizing soil with phos- 
phates depends in major portion upon the coagulating effects 
of the former, which thereby antagonize the adsorption of 
the phosphoric acid by the soil colloids. Unfavorable ad- 
sorption effects undoubtedly come to pass when the soil 
contains too much colloid material. These equilibrium 
considerations lead to the same conclusion which the prac- 
tical workers in agriculture have so long held. Under 
otherwise constant conditions, a medium colloid content gives 
greatest fertility. But this medium colloid content may have 
different absolute values, depending upon the concentrations 
of the nutritive materials present — in other words, depending 
upon the chemical composition of the soil and the individual 
needs of different plants. It seems to me that this view best 
coordinates the numerous and apparently contradictory 
findings of different students of the question covering the 
relationship between soil colloids and its fertility. 
In passing I should like to mention the interesting chemi- 
cal consequences, like the so-called adsorption decompositions, 
which often follow adsorption in soils. Many years ago, 
J. M. van BEMMELEN showed that gels absorb the potassium 


SCIENTIFIC APPLICATIONS 165 


from potassium sulphate solutions and not the sulphate — 
this being followed by the appearance of free sulphuric acid 
in the solution undergoing adsorption. These specific ad- 
sorption phenomena which in their turn may be followed 
by tremendous secondary chemical changes undoubtedly 
play a great role in the dynamics of the soil.! 


§13. 


I have already used up the major portion of lecture time - 
and yet am only now coming to perhaps the greatest and 
most interesting of all the scientific applications of colloid 
chemistry. I refer to those made in biology and medicine.’ 
Colloid chemistry is the promised land of the biological 
scientist, and it is almost impossible for the enthusiastic 
colloid chemist not to become poetical in this region. 

As you know, the elements necessary for life may be 
gathered together under the chemical headings of the 
proteins, the lipoids, the salts and water; but the physical 
and the physico-chemical conditions necessary for life can- 
not be more accurately or more concisely summed up than 
in the words all life processes take place in a colloid system. 
The colloid state is the means of integrating biological 
processes. More correctly expressed, only those structures 
are considered living which at all times are colloid in com- 
position. 

It is self-evident that because of the close association 
between colloid chemistry and biology the number of in- 


1 The relations of agriculture to colloid chemistry are detailed in the ex- 
tensive volume of P. ExrenBrerG, Die Bodenkolloide, 2te Aufl., Dresden, 
1918, as well as in the shorter but highly to be reeommended text of G. Winc- 
NER, Boden und Bodenkolloide in Kolloidchemischer Betrachtung, Dresden, 
1918, a second edition of which is in preparation. 

2 It is impossible to list a series of papers which will cover adequately the 
many relations of colloid chemistry to biology and medicine. For a first 
orientation in this field, see H. BecHHoLp, Colloids in Biology and Medicine, 
trans. by BuLtowa, New York, 1919, where numerous references will be found. 
The physical peculiarities of living matter with due emphasis upon its colloid 
nature are discussed in L. RuumBuER, Das Protoplasma als physikalisches 
System, Wiesbaden, 1914. Larger volumes dealing with this general subject 
are those of R. Hosrr, Physikalische Chemie der Zelle und Gewebe, Leipzig, 


166 COLLOID CHEMISTRY 


dividual colloid-chemical laws which hold in biology must 
be enormously great, for since organisms are merely special 
instances of colloid systems, there can exist no biological 
problems in which colloid chemistry must not play some 
part. The colloid-chemical point of view permeates biology 
from its beginnings in causal morphology to its endings in 
chemical physiology. Bacteriologists, physicians, students 
of experimental morphology, plant physiologists, all are 
interested in colloid chemistry and its development. The 
biologists find colloid chemistry useful to their ends as is no 
other science. It has not, however, been pressed into their 
hands ready-made; from the earliest days they have them- 
selves furthered the principles of pure colloid chemistry and 
then applied them to their specific problems. Nothing 
demonstrates better the close relations between the two 
sciences than the fact that a large number of colloid chemists 
entered their fields from biology or through biology. I 
need but mention A. Fick, C. Lupwia, F. Hormutstrer, Wo. 
Pauui, W. M. Bayuiss, M. H. Fiscorr and F. Bottazzz. 
And even the newest chapters in colloid chemistry are 
indebted for their rapid and magnificent growth in no small 
fashion to the interest and enthusiastic cooperation of the 
biological colloid chemists. 


814. 


I choose arbitrarily when from the wealth of the biolog- 
ical applications of colloid chemistry I select a few examples. 
Let us begin by asking how it is proved that living sub- 
stance is itself colloid. Chemical analysis shows that the 
characteristic building block of organic material is protein. 


1912; N. Garpuxow, Dunkelfeldbeleuchtung in der Biologie, Jena, 1911; 
F. Borrazzi, Handbuch der vergleichenden Physiologie, Jena, 1913. <A par- 
ticularly important volume is that of Martin H. Fiscner, Gidema and 
Nephritis, 3d edition, New York, 1921, in which not only medical but many 
non-medical problems of biological interest are discussed; see also his Fats 
and Fatty Degeneration, New York, 1917, and his Soaps and Proteins, New 
York, 1921. 


SCIENTIFIC APPLICATIONS 167 


But, as my previous lectures showed, it is this very sub- 
stance which is most typical of the colloids, belonging, as it 
does to the class of the hydrated emulsoids. The fact that 
great numbers of experiments in colloid chemistry are made 
with fresh egg albumin, with blood serum, with muscle juice, 
indicates that the proteins exist in colloid form in the living 
organism and that they are not ‘‘produced”’ by chemical 
methods. Moreover, it is possible to coagulate all or parts 
of a living cell (like the flagellee of bacteria), and the means 
employed to this end and the results obtained are identical 
with the coagulation of proteins in test tubes. Finally — 
and this is perhaps the simplest direct proof of its colloid 
nature — living matter may be studied under the ultra- 
microscope. ‘This experiment can, however, not be carried 
on very long, for the intense light needed soon kills the 
organism with the exhibition of the signs of coagulation. 
When the colorless plasma of an alga cell! is thus studied, it 
is seen to consist of a mixture of numerous particles of differ- 
ent sizes, of which a large number are typically colloid. It 
is of special interest that many of these particles are in 
active motion. ‘The particles approach each other, separate, 
coalesce to form larger particles, disappear entirely — indeed 
he who observes this ultramicroscopic picture of a living 
cell for the first time will perhaps be inclined to hold that 
the ‘‘true”’ life of any cell is not to be seen except ultra- 
microscopically. In this he is to a certain extent at least 
in error, for the movement of the colloid particles in living 
~ matter is nothing more than the same movement as observ- 
able in any sol showing BROWNIAN movement. ‘The ultra- 
microscope shows the plasma of living cells to be a mizture 
of hydrosols of different degrees of dispersion. This con- 
clusion, therefore, bears out the results of chemical analysis. 
There is, moreover, no contradiction between this finding 
and the fact previously emphasized that ultramicroscopic 
analysis often fails to establish the colloid character of the 
biocolloids. This is because of their great hydration. 
1 See N. Gamnuxow, Dunkelfeldbeleuchtung in der Biologie, Jena, 1913. 


168 COLLOID CHEMISTRY 


It may now be asked whether this mixture of hydrosols 
exists in a form in which the dispersed particles, like the 
proteins or lipoids, float about in the dispersion medium, or 
whether through secondary rearrangement these colloid 
particles may go to form microscopically visible structures. 
The biologists among you will be well aware that a union 
of particles into a network or a honeycomb is often seen 
in living protoplasm. There are some authors—as O. 
Butscuui — who have held that such a subcolloid structure 
is characteristic of all living matter. Can the colloid-chemi- 
cal laboratory explain how such microscopic structures come 
to pass in living protoplasm? It can. N. BrtyErIncK dis- 
covered the interesting phenomenon that when two liquid 
colloids like gelatin and starch are mixed in definite con- 
centrations, the resulting mixture of hydrosols does not 
show the particles of the two to be dispersed in colloid form 
uniformly throughout the liquid but that one of the colloids 
divides itself in droplet form into the second. The mixture 
in consequence assumes a net or honeycomb-like appearance. 
The structure thus produced is obviously analogous to that 
seen in living matter when studied microscopically or ultra- 
microscopically. 

$15. 


A question much discussed in general biology and an- 
swered through colloid chemistry asks regarding the physi- 
cal state of living matter. Is protoplasm solid or liquid? 
Time has shown that the form of this question is again a 
wrong one. Protoplasm is neither solid nor liquid when 
compared with typical solids or liquids. Its physical 
peculiarities are those of a hydrated emulsoid which may 
show all degrees of fluidity ranging from those values which, 
on the one hand, are characteristic of a normal liquid to 
those which, on the other, are characteristic of a solid. -As 
gelatin, depending upon its temperature and its concentra- 
tion, may show all states from a liquid to a solid, just so may 
protoplasm. As a dilute gelatin gel — and we shall see 
shortly that living protoplasm is just such a dilute colloid — 


SCIENTIFIC APPLICATIONS ——-: 169 


unites within itself the properties of a liquid and of a solid, 
just so does living matter show properties which at one time 
make us think it fluid and at another solid. Protoplasm 
shows, for example, capillary phenomena, protoplasmic 
streamings, vacuole formation, throws out pseudopods, and 
its separated portions form droplets.! All these are the 
properties of liquids. Upon the other hand, protoplasm 
shows a plasticity and a maintenance of form which is seen 
only in solids. . An ameba deformed through pressure tends 
gradually to resume its spherical form; and slight but per- 
sistent pressure will give the embryonic cells of frog eggs 
some other than a spherical shape, and this will persist for 
hours. | 

It would seem that living matter is constantly oscillating 
between the extremes of a solid gel and a liquid gel. Such 
oscillations probably underlie ameboid mot‘on and are 
the cause for the appearance and disappearance of the 
numerous structures seen at different times in the life cycle 
of the cell. The German investigator L. RHUMBLER, who 
has studied the physical properties of living matter. in 
masterly fashion, came to the conclusion that only a spe- 
cially dispersed structure — a ‘‘spumoid”’ structure, as he 
calls it — can account for this remarkable combination of 
physical properties. He himself emphasizes, however, that 
this structure must be of the same kind as that possessed 
by any hydrated emulsoid. In reality the physics of the 
two is the same. 

$16. 


Closely related to the physical properties of living matter 
is its great water content. It is not generally recognized how 
very great is this proportion of water in living matter. 
More than half our body weight, for example, is water and 
marine alge and jelly fish hold as much or more than 
ninety-six percent. It is certainly remarkable that these 
organisms not only hold their shape, but move, swim, eat 


1 See L. RuumBier, Das Protoplasma als physikalisches System, Wies- 
_ baden, 1914. 


170 COLLOID CHEMISTRY 


and make love — and this with ninety-six percent of water 
in their affection. These things are made possible through 
colloid chemistry, for only colloid systems — in other words, 
the gels — can hold their shape when thus rich in water. 
These remarks explain why the biological question of how 
this water-holding power is brought about and how it is 
regulated under different circumstances becomes a problem 
in colloid chemistry. 

No doubt you know that in the heydey of the classical 
physical chemistry of the solutions, other and non-colloid, 
namely, osmotic forces, were called upon to explain the 
absorption and the movement of water in organisms. It 
was assumed that the cell membranes found in the tissues 
of the higher animals acted as osmotic membranes in that 
they gave passage to water but. did not permit dissolved 
substances like the various salts to pass through them. In 
this way through concentration differences an osmotic move- 
ment of water was brought about into the cell and thus its 
cell turgor was maintained. The modern developments of 
cellular physiology have shown more and more clearly that 
this réle of the osmotic forces has been greatly overestimated. 
It may now be said that there exist but exceptional instances 
in which may be discovered any fairly complete analogy 
between the laws of osmotic pressure and those which govern 
the absorption of water by a cell. Even though I would not 
hold to the extreme view that osmotic processes play no réle 
whatsoever in the processes of water absorption by living 
organisms, unprejudiced consideration of the facts compels 
the conclusion that besides these, or better expressed, far 
transcending these in importance, entirely different forces 
determine the water content of an organism. Not the 
osmotically active, molecularly dispersed constituents of a cell 
(more particularly the salts therefore) but the plasma colloids 
are primarily responsible for the water content of the luing 
organism and for the changes which this shows. ; 


1 Important new work bearing on this question is that of W. von MOELLEN- 
porFF, Koll.-Zeitschr., 23, 158 (1918) who shows that osmotic processes (in 


SCIENTIFIC APPLICATIONS DAL 


Looked at now this conclusion, which in broad form was 
first drawn and discussed by Martin H. Fiscuer,! seems 
almost self-evident. For not only do we know from labora- 
tory experiments that the emulsoids obtained from living 
organisms are able to hold enormously large amounts of 
water just as can the living organisms themselves, but we 
know also that this capacity for holding water, as measured, 
for instance, through viscosity changes, can be tremendously 
modified by apparently trivial and widely different types 
of changes in surroundings. The influence of electrolytes 
(such as acids, bases and salts) is so great that variations 
in their concentration within biological limits results in 
marked variations in the water content. I should like to 
emphasize this by citing an example. As I have already 
shown you, acids in moderate concentration increase tre- 
mendously the amount of water absorbed by gelatin, fibrin 
or egg albumin. The influence of the hydrogen ion is so 
enormously great that the presence of even such a “‘weak”’ 
acid as carbonic acid brings about a distinct increase in 
swelling. As Wo. Pauui and R. Curart have shown, the 
amount of water absorbed by a gelatin plate which is kept 
in freshly distilled water is much less than that of one kept 
in a distilled water exposed to air containing a little carbon 
dioxid. This increase in swelling may be used as an indica- 
tor for the presence of hydrogen ions. These things show, 
at the same time, how very easily, through variations in the 
chemistry of living organisms, changes may be brought 
about in living matter which will alter its water content. 
We shall return to this question shortly. 


so far as they appear at all) can come to pass in certain types of cells only 
within their bodies. The osmotically active ‘‘membranes”’ are to be looked 
for in the surfaces which separate the droplets of hydrated biocolloid from 
the cell sap. But this ‘‘microdsmosis” or ‘‘dispersed type of osmosis” 
will probably not follow the classical and simple laws of osmosis as observed 
in macroscopic systems but exhibit transition characteristics tending to 
class it with the phenomena of swelling and hydration. 
1 See the literature cited in the footnote on page 166. 


ive COLLOID CHEMISTRY 


$17. 


Besides these structural and physical peculiarities common 
to living matter which are newly illuminated or explained 
through colloid chemistry, there exist many close relations 
between colloid chemistry and the more purely chemical 
and physico-chemical reactions which are characteristic of 
living matter. It has often been asked, for example, how 
it is possible that so many different reactions may take place 
side by side in a cell (a structure which consists so essentially 
of liquid) without all running together and yielding chaos. 
The experiments with colloid mixtures mentioned above 
show that two colloid substances — even when divided into 
the same dispersion medium — need not mix with each 
other, but can continue to exist side by side in the form of 
microscopic droplets. The great variety of microscopic and 
ultramicroscopic structures observable in living matter 
leads to the conclusion that the different chemical con- 
stituents of the protoplasm— more particularly those 
existent in colloid form — may in similar fashion exist side 
by side without mixing. As F. Hormetsrsr has indicated, 
we may imagine each of the individual droplets to be a tiny, 
special laboratory, in which, undisturbed by the surround- 
ings, some one or certain few reactions take place. Such a 
localization of chemical processes within the mixture would 
be aided by the fact that a large part of the reacting sub- 
stances and of the reaction products are colloid and there- 
fore do not on their own accounts tend to diffuse and So 
mix themselves with neighboring substances. 


$18. 


But the colloid state plays a great réle in the chemistry of 
living matter in yet another direction. We have repeatedly 
emphasized that adsorption processes must play a great réle 
in catalysis, that, in fact, many of its features must be held 
to be the direct consequences of concentration increases 
brought about in surfaces. A colloid mixture of the type 


SCIENTIFIC APPLICATIONS 173 


of protoplasm must therefore offer peculiarly favorable con- 
ditions for the play and for the acceleratian of chemical 
reactions. It is consequently not to be wondered at that 
those substances which are to be counted among the most 
fundamental of the constituents of living matter, namely, 
the ferments, are known for the most part only in colloid 
form. Living matter seems to be a meeting ground for 
adsorption effects and colloid catalyses. 

Allow me, after this general survey, to touch upon a series 
of special biological problems in which the colloid-chemical 
point of view has brought much light. The choice must 
again be arbitrary and my review most superficial. For 
further details I direct you to the special articles which deal 
with these problems.! 


$19. 


Colloid chemistry brings us light in even those first of all 
biological processes which are concerned with the formation 
of new organisms, namely, the phenomena incident to 
fertilization and the early development of the embryo. 
You perhaps know that the stimulus to the development of 
a sea-urchin egg, for instance — it matters not whether this 
be brought about by sexual fertilization or by so-called 
artificial means — is characterized morphologically by the 
formation of a so-called astrosphere. Rays of concentrated 
plasma appear either in the immediate vicinity of the 
nucleus of the fertilized egg, or, it may be, in other 
portions of the egg plasma. These rays then act as 
centers toward which the products of the divided nucleus 
move. I cannot go into details, but in spite of the specific 
variations which appear in different animals, what I have 
described is constant in all fertilization and cell division. 

Closer study of the problem proves that this formation 
of the astrosphere represents a special form of coagulation of 
the plasma colloids, and microscopic observation suffices to 


1 See the second footnote on page 165, as well as numerous articles in the 
Kolloid-Zeitschrift in which references to other striking papers may be found. 


174 COLLOID CHEMISTRY 


show that we are dealing with a localized collection of water- 
poor and coarsened plasma. We deal, in other words, with 
the conversion of a sol into a gel. Micro-dissection proves 
this without the question of a doubt, for the astrospheres 
may be pulled, as more solid masses, out of the relatively 
fluid egg plasma (G. L. Kitz, R. CHAmBERs). 

The conclusion that this phase of fertilization represents 
a coagulation process may be proved by yet other means. 
Those of you who have followed the well-known studies of 
Jacques Lors and of other investigators of the problem 
of artificial parthenogenesis will know that the unfertilized 
eggs of sea-urchins or star-fish may be made to develop by 
many different means. Not only does treatment with acids, 
alkalies or specific ions lead to this result, but the water 
extraction incident to the effects of neutral salts is also 
effective. ‘Temporary exposure to high and low tempera- 
tures, exposure to other colloids (as the serum of higher 
animals), treatment with organic liquids like benzol or toluol 
and even mechanical treatment (such as shaking, rubbing 
or brushing) accomplishes artificial development in the 
unfertilized eggs of many organisms. What have all these 
methods in common? If you will recall what I said yester- 
day regarding the alterations in the colloid state that may 
be brought about through trivial external changes, it will 
be clear to you that all these methods for inducing artificial 
parthenogenesis are such as lead to changes in the colloids — 
more especially to their coagulation. All the listed methods, 
to which more might be added, leading to the development of an 
egg, serve also to produce coagulation in protein sols. Con- 
versely we may say that we hardly know a method of pro- 
ducing such protein precipitation which when properly used 
may not also be employed to start artificial development.? 

This coagulation theory of fertilization receives pretty 
support through the possibility of causing in colloids and 
colloid mixtures a localized and oriented coagulation which 


1 Martin H. Fiscoer and WouraanG OstTwALp, Pacey Archiv. f. d. g. 
Physiologie, 106, 229 (1905). 


SCIENTIFIC APPLICATIONS 175 


in structure is strikingly like the astrospheres observed in 
developing cells. Fig. 42 is a picture of such an artificially 
produced astrosphere taken from the work of O. BurscH.r 
and made at a time when the colloid-chemical theory of 





Fia. 42. 


fertilization which I have sketched to you had not yet been 
born. The biologists among you will grant its striking 
similarity to the real thing. 

The primary process which leads to development in an 
egg is seen, therefore, to be a colloid-chemical one and of the 
nature of the transformation of a sol into a gel. But please 
do not misunderstand me in the matter. I would not have 
you think that this explains everything that there is to the 
process of fertilization. Many different chemical processes, 
as those of increased oxidation, for example, accompany the 
astrosphere formation, but these appear only after the colloid 
changes which I have described have started the process. 
That which starts development is colloid-chemical. 

But not only is the beginning of development in a metazoon 
egg capable of a surprisingly thorough experimental colloid- 
chemical analysis but so is the process of cell division and 
the increase in the number of cells in unicellular organisms. 


176 COLLOID CHEMISTRY 


The experiments of J. Spex! need special mention in this 
connection. ‘This investigator has shown that the rate of 
multiplication of infusoria (paramecia) can be greatly in- 
creased, ten to twenty times in fact, through the addition 
of salts which like lithium chlorid favor swelling; salts like 
the sulphates, on the other hand, which inhibit swelling 
decrease the rate even when added in amounts not toxic. 
Even so complicated a phenomenon as that of gastrulation 
SPeEK found to be capable of theoretical and experimental 
colloid analysis on the assumption that the phenomenon is 
essentially one of swelling. In fact this enthusiastic investi- 
gator seems to have utilized this single colloid-chemical 
change perhaps too exclusively and so ignored, for example, 
other simultaneously possible colloid changes as the co- 
agulative phenomena already referred to above.2 One 
needs to bear in mind that protoplasm represents a mixture 
of colloids and that one and the same electrolyte for example 
may act in exactly opposite fashion upon two different 
colloids, producing the flocculation (dehydration) of one, 
while making a second swell.* It is necessary to emphasize 
these facts in order to make sure that a just appraisal is 
made of the importance of each of the various colloid- 
chemical changes that may play a réle. It represents, how- 
ever, a happy state of affairs when it is seen that differences 
of opinion exist between workers only as to the relative 
values to be attached to various colloid-chemical changes 
in the analysis of a biological phenomenon; that the bio- 
logical phenomenon is a problem which involves only colloid 
changes — this none of the workers doubts.. 


1 J. Spex, Kolloidchem. Beihefte, 10, 259 (1918); 12, 1 (1920) where refer- 
ences to the literature may be found. 

2 I hope at another time to bring proof for this conclusion publicly; thus 
far I have done so only in correspondence. 

3 The phenomenon of ‘‘cloudy swelling” as observed in the cornea may be 
cited as an example. According to the investigations of Martin H. FiscHEr’ 
cloudy swelling results from the action of an acid (or a similarly acting sub- 
stance) simultaneously but in opposite directions upon different corneal 
colloids. See the text further along. 


SCIENTIFIC APPLICATIONS 177 


§20. 

Swelling phenomena also play a large part in the various 
phenomena characteristic of growth. Chemical analysis 
shows frog larve, for instance, to owe their enormous 
changes in weight up to the time of their metamorphosis to 


AY 
6000 M‘ 


oe 


th 
Olay elon. 


! 
1 
! 
| 
| 
1 
1 
1 
\ 
| 
1 
| 
! 
\ 
| 
! 
\ 
| 
\ 
! 
! 
! 
I 
1 
1 
! 
! 
| 


/’ 


1000 


ne se 
| organic guste Mt 
50 


—— A oe OE gis 
10 = 20 30 = 40 50-660 7 80 90 Days 
Fig. 43. — Increase in weight during growth of frog larve. 


land animals to mere changes in the amount of water ab- 
sorbed. As shown in the curves of Fig. 48, the increase in 
amount of solid substance is so slight that at the time when 
the frog takes to land, it consisto of ninety-three percent of 
water. 


1 See A. Scuaper, Arch. f. Entwicklungsmechanik, 14, 356 (1902); also 


Wo.LFcanG OstwaLpD, Uber die zeitlichen Eigenschaften der Entwicklungs- 
vorginge, 49, Leipzig, 1908. 


178 GOLLOID CHEMISTRY 


These same facts hold for the growing parts of plants. 
A change in the osmotically active constituents of a growing 
part great enough to account for these enormous water 
absorptions is unknown. We know, however, that the 
erowing parts of many plants are acid. But as previously 
pointed out, acids enormously increase the water absorbing 
powers of various colloids even when present in only minimal 
concentrations. These things point clearly enough to the 
importance of swelling phenomena in growth processes. 

When the developing frog becomes a land animal, it loses 
much water. But this may also be explained colloid- 
chemically, for, as I have previously emphasized, a gel 
- under ordinary experimental conditions has a different and 
lower swelling point when in equilibrium with water vapor 
than when in equilibrium with fluid water. Colloid chem- 
istry is also interested in the fact that desert plants often 
show an acid reaction — a circumstance which would per- 
mit them not only to take up more water, but also to hold 
better such as has been absorbed against the forces leading 
to drying. The diurnal changes in the reaction of plants are 
probably also followed by similar variations in water ab- 
sorption capacity. 


$21. 


Another problem to the solution of which colloid chemis- 
try has been called is that of the nature of the muscular 
contraction. I cannot go into details, but I should like to 
point out that the electrical, chemical, mechanical, optical 
and other changes incident to the muscle contraction can 
all be best understood in the terms of colloid chemistry.’ 
The essence of the muscular contraction seems to reside in a 

1 See Martin H. Fiscuer and W. H. Srrmrmann, Koll.-Zeitschr., 10, 
65 (1912); also Woireana PAUvLt, Kolloidchem. Beihefte, 3, 361 (1912) 
and the recently issued monograph of O. von Furtu, Ergebnisse der Physi- 
ologie, Wiesbaden, 1920. To von Furru’s list of references should be added 
that the first quantitative studies on the effects of acids in bringing about the 
swelling of gelatin date from Woutraane OsTWALD, Pfliiger’s Arch., 108, 
563 (1905). 


SCIENTIFIC APPLICATIONS 179 


transport of water from certain of the structural elements 
making up the muscle to certain other contractile elements. 
‘Differently expressed, a migration of water occurs from one 
colloid to a second making up this tissue. This migration 
is brought about through a production of acids (more 
especially of lactic acid) in the muscle. In this we see again 
the so-widely distributed and so exceedingly active influence 
of the hydrogen ions upon water absorption by a biocolloid. 
That the phenomena of swelling as observed outside of the 
body may occur with a rate and to an extent demanded by 
a colloid-chemical theory of the muscle contraction — this 
I showed you yesterday in discussing the swelling of gutta- 
percha leaves and of gelatin. In discussing swelling I also 
emphasized that an amount of mechanical energy is lib- 
erated which is entirely adequate to explain the mechanical 
phenomena incident to the contraction of a muscle. 


$22, 


Of the many other problems in physiology which seem 
accessible to colloid-chemical analysis, I can only touch 
upon that of secretion. The physiologists among you will 
know that what physiology seeks is an understanding of the 
nature of the ‘‘driving”’ forces which bring about the secre- 
tion of water by a cell or tissue — at times even against the 
existence of a counter-pressure like hydrostatic or osmotic 
pressure. Perhaps more than in any other chapter of 
physiology do we in this problem of secretion still speak of 
‘‘vital”’ forces. Even after utilizing the newest concepts 
of physical chemistry in addition to the older ones of filtra- 
tion, diffusion, etc., we still have much left to be explained. 
Here again, colloid chemistry is acquainted with forces 
which, so far as we can see, are fully able to meet the require- 
ment that the forces producing secretion must be essentially 
independent of hydrostatic and osmotic pressure differences, 
while it makes clear at the same time the nature of a number 
of the phenomena which commonly accompany such secre- 


180 COLLOID CHEMISTRY 


tion. I refer to the phenomena of secretion observable in 
simple colloids, and discussed in the last lecture under the 
heading of syneresis. 

I ask you to recall that every secretion springs from a 
colloid matrix, and that the secretion contains not only 
water but colloids and salts and these of the kind present 
in the secreting tissues themselves. Even the most watery 
of the secretions, like the urine, contains a series of non- 
dialyzable substances, the so-called “‘colloid nitrogen.” 
What is true of secretion is also true of syneresis. The 
serum squeezed off contains not only water but also colloids 
and salts and these in proportions which need in no sense be 
identical with those existent in the secreting gel. In synere- 
sis in colloids, as in physiological secretion, both the amount 
and the composition of the serum given off varies not only 
with the kind of colloid but with the kind and the amount 
of the material contained in it, etc... But what is most 
important is that the syneretic secretion of fluid is not 
determirfed through osmotic or hydrostatic, pressure differ- 
ences, but is dependent upon forces existent “within”? the 
gel itself — upon forces, in other words, which lead to 
changes in its ‘‘internal state.” With these suggestions I 
must let the matter rest. 


§23. 


Another physiological problem much studied recently 
is that of vital staining, in other words, the taking up of 
dyes by living cells. It has become increasingly evident 
that the degree of dispersion of the dye is a factor of prime 
importance in bringing about positive results.? As a rule 
only molecularly or highly dispersed dyestuffs can be taken 
up, the plasma film surrounding cells seeming to act like 
an ultrafilter. 

1 See the footnotes on pages 98 and 99 as well as the accompanying text. 

2 See especially W. Ruuianp, Koll.-Zeitschr., 12, 113 (1912); 14, 48 


(1914), where the literature is cited; also the papers of W. SCcHULEMANN and 
W. von MogELLENDORFF in the more recent volumes of the Kolloid-Zeitschrift. 


SCIENTIFIC APPLICATIONS 181 


I would like to add a word here regarding our methods 
of fixation and staining of dead tissues. Some twenty 
years ago the biologists were much frightened when the 
botanist A. FiscHer pointed out that many of the struc- 
tures found after such treatment are ‘‘artifacts.” Fixing 
and staining reagents bring about dehydration and coagu- 
lation effects — in other words, colloid-chemical changes — 
in the state of the tissue colloids. It is undoubtedly true 
that many things may be seen in such fixed tissues which 
it would be wrong to say exist in living protoplasm; on 
the other hand, it would be just as wrong to hold that 
such fixation methods can tell us nothing whatsoever re- 
garding the structure of living matter. A rational fixa- 
tion and staining technique can apprise us of the character 
of the changes wrought by fixation and staining reagents 
in test tube experiments upon such highly hydrated mix- 
tures as are presented by the biocolloids; and it is per- 
fectly safe to apply the conclusions won in this fashion to 
the related problems of histology and biology. ‘The com- 
parative experiments of this type which have been made in 
masterly fashion, for example, by G. MAnNn,! and more 
recently by R. E. Lizsecane form the foundation stones of 
a rational histology. Obviously even the normal micro- 
scopic structure of living tissues is the result of changes in 
state of the biocolloids, wherefore detailed study of these, 
such as is presented by protein sols in a test tube or on a 
slide, can in this fashion be used for the interpretation of 
the ways and means by which normal structure is produced. 


§24. 


These and associated colloid-chemical studies serve in 
this way to contribute to a science of which, I admit, I 
speak with reluctance, even though it is the crown of all 
biology. I refer to synthetic biology, the science of the 
artificial production of living things. Since the synthetic 


1G. Mann, Physiological Histology, Oxford, 1902; see the review v of this 
book in Koll.-Zeitschr., 2, 153 (1907). 


182 COLLOID CHEMISTRY 


production of urea by Lirpia and WOHLER, we have been 
familiar with a synthetic biochemistry. It is today an easy 
matter to produce in the laboratory substances and re- 
actions which are commonly seen only in living organisms. 
By comparison we are still much in the dark regarding a 
sister science, that of synthetic biophysics. Even when we 
succeed in producing ameboid movements in drops of 
liquid or in colloid mixtures, or when we discover methods 
whereby non-living matter can be made to build protec- 
tive coverings for itself, to exercise choice in the ‘taking 
up of nutritive materials, we incline to call these analogies 
to biological processes ‘‘imitations’’ of the processes and 
thus to cheapen the value which we set on them. But such 
experuments are experiments in synthetic biophysics and of 
exactly the same significance as the synthesis of urea or ca- 
talysis by colloid metals for synthetic biochemistry. 7 

Like the chemistry of organized substance, so must its 
physics be analyzed into unit processes and through 
gradual rebuilding from these be resurrected into a syn- 
thetic biology. ‘Trustworthy results will, of course, be 
obtained only through systematic study. It would be 
most unscientific, for example, to call certain precipitates 
primitive organisms because they look like such. A 
synthetically produced organism must, naturally, show all 
the fundamental characteristics of organized matter at 
one and the same time. But in spite of the great dis- 
tance still to be traversed before such a goal is attained, 
there can be no doubt that through a proper combination 
of individual chemical and physical processes of the types 
observable in organisms, the attempt to reach such a goal 
represents an entirely scientific problem. In the still 
much-neglected biophysics of the colloids there is already 
at hand a wealth of suggestive material. 3 


$25. 


I must in conclusion give you a hasty view of the appli- 
cations of colloid chemistry to medicine. Obviously the 


SCIENTIFIC APPLICATIONS 183 


number of possible applications here is just as great as in 
the biology of the normal organism, for pathological changes, 
too, take place only in that colloid foundation in which 
all normal life processes occur. Just as normal causal 
biology must be edited — must be rewritten, in fact — in 
the terms of colloid chemistry, even so must pathology be 
rewritten. Time does not permit me to enter into many 
details, but a few interesting examples will illustrate my 
point. 

Closely associated with the general problem of how an 
organism holds its normal water content is that of the ways 
and means by which it holds more than this, as is the case 
in the pathological phenomena of edema and its various 
clinical subheadings. As in the case of the normal ab- 
sorption of water by the normal organism, the tissue col- 
loids again play a chief réle in this pathological problem 
as demonstrated in the fundamental investigations of 
Martin H. Fiscumr.! Changes in the water-holding ca- 
pacity of the tissue colloids are responsible for edema and 
an abnormal production or accumulation of acids in the 
involved tissues again appears to be the main agent favor- 
ing the swelling, even though it is possible that the hy- 
drating effects of other substances like the proteolytic 
ferments (W. Gins) may also act in this direction. An 
abnormal production or retention of acids can be assumed 
or proved to be the primary, etiologic cause in most cases 
of edema. Acids are produced, for example, whenever the 
normal processes of oxidation are inhibited, as through 
the presence of different poisons, through a shutting off 
of the circulation (passive congestion), through anatomical 
changes in the organs necessary for the maintenance of 
a proper circulation, or when in consequence of a flea bite 
or a bee sting formic acid is introduced locally into a tissue. 
Experimentally ‘‘artificial flea bites” can be produced very 

1 See the numerous papers of Martin H. Fiscuer and his collaborators 


in the Kolloid-Zeitschrift and the Kolloidchemische Beihefte as well as his 
(Edema and Nephritis, third edition New York, 1921. 


184 COLLOID CHEMISTRY 


nicely by pricking a gelatin plate with a needle dipped in 
formic acid and then placing the gelatin plate in a little 
water (demonstration).! : 

The correctness of the view that the water-holding prop- 
erty of the colloids and changes in their state determine 
both the normal and the pathological water content of 
tissues can also be demonstrated in the following fashion 
(demonstration). I have left untouched the experimental 
apparatus with which I showed you the influence of elec- 
trolytes upon the swelling of gelatin. I have merely placed 
in the different solutions beside the gelatin discs, whole 
organs, sheep eyes, frog legs, etc., and have left them there 
for a number of hours. If you will look at these experi- 
ments you will note that the influence of these different 
electrolytes upon the swelling of these organs parallels 
completely the influence of these same substances upon the 
swelling of the gelatin discs. You note that in the acid 
and in the alkali there is an enormously greater increase 
in the size of the organs than in the pure water — in other 
words, they have absorbed more water in the acid and 
alkali than have the organs left in the pure water. On 
the other hand, where a proper salt has been added, a dis- 
tinct shrinkage has occurred (Fig. 44). When we touch 
the organs which have swelled in the acid we become con- 
scious of the same feeling which edematous organs give us. 

In passing let me emphasize that just as the addition 
of salt to gelatin or fibrin swelling in the presence of an 
acid reduces the amount of the swelling or suppresses it 
entirely, just so has this principle been successfully em- 
ployed in the reduction of clinical forms of edema.2 

It has been argued against this colloid-chemical theory 
of edema that it does not suffice to explain how the ex- 
cessive accumulations of fluid which are often found be- 


~ 1 See Martin H. Fiscupr, (idema and Nephritis, third edition, 241 and 
733, New York, 1921. 


* See the papers and books of Martin H. Fiscuer and his collaborators. 
See also Martin H. Fiscuzr, Kolloidchemische Beihefte, 4, 343 (1918). 


SCIENTIFIC APPLICATIONS 185 


tween the cells and in the body cavities are to be explained. 
But this phenomenon whi chwe are wont to see particu- 
larly in the latter stages of edema, is also easily explained 
colloid-chemically. The spontaneous secretion of such 
liquids, which, as you know, are often rich in albumin, 








Extruded Vitreous 
steamy externally 


Bluish 





~~ Intensely white 


=I Intensely (yellowish) white 
fx (Leukoma) 







White externally 


(Yellowish) white 


White nucleous 


White 


C 


Fic. 44. — Swelling of sheep eyes according to Martin H. FiscHer. 


(a) the normal eye (c) in HCl plus Mg(NOs)o 
(b) in HCl (d) in HCl plus FeCl; 


is the analogue of what we call syneresis in colloids and 
may be expected to appear in particularly marked form 
whenever the gels from which they are squeezed off are 
particularly rich in water. As I pointed out before, the 
amount of fluid thus squeezed off by any hydrophilic col- 
loid like gelatin increases with increase in the water con- 
tent of the gel. 

Less just still is another objection which has been raised 
against FiscuEr’s theory of edema. It is pointed out that 


186 COLLOID CHEMISTRY 


the concentration of the ‘‘free’’ acid or of the ‘“‘free”’ hy- 
drogen ions is rarely higher in edematous tissues or fluids 
than in normal ones because, as generally held, the organ- 
ism regulates the ‘‘reaction”’ of its tissues through buffer 
mixtures, etc. This objection is to be answered by saying 
that the increased water absorption is of course not to be 
attributed to any ‘‘free’’ acid which may be found but, 
contrariwise, to the ‘‘non-free”’ acid, in other words, to the 
acid which has been bound by the tissues. Only the elec- 
trolyte bound to a colloid either through adsorption or chem- 
ical union can alter its state. The nature of the equilibrium 
between the bound acid and the free is still a question which 
requires quantitative study. If it is assumed that in the low 
acid concentrations involved we are dealing with adsorp- 
tion then we get an immediate explanation of why union 
with a relatively large amount of acid goes hand in hand 
with the presence of only a minimal amount of free acid; 
for in low concentrations, as you know, adsorption is prac- 
tically complete. It needs also to be emphasized that the 
swelling in acids is in no sense a main function of the con- 
centration of the hydrogen ions. Gelatin, for example, 
swells more in the weakly dissociated acetic acid than in 
the practically completely dissociated sulphuric acid, while, 
on the other hand it is precipitated (that is to say, dehydra- 
ted) through picric acid which is another highly dissociated 
acid. Here as in other purely physico-chemical reactions 
it is not merely a matter of the hydrogen ion concentration 
but equally important or more important is the concen- 
tration of the non-dissociated acid molecules. 

Inspection of an eye which has been permitted to swell 
in an acid shows the eye to be in a state which clinically 
we would call glaucomatous. The cornea is, moreover, 
steamy or opaque (Fig. 44). In the terms of colloid chem- 
-istry we deal with an increased water absorption by some 
of the ocular colloids while the clouding represents an 
acid coagulation of a second group of the biocolloids. In 


SCIENTIFIC APPLICATIONS 187 


the case of this second group the acid concentrations em- 
ployed do not bring about an increased but rather a de- 
creased hydration and coagulation of the involved ma- 
terials. Since our tissues represent a mixture of very 
different types of colloids, this double effect is readily 
intelligible colloid-chemically. 7 


§26. 


As also shown by Martin H. Fiscuier, these combined 
swelling and coagulation processes reappear in many cases 
of nephritis... The method of handling these cases thera- 
peutically through the introduction of salt solutions — 
more particularly of solutions alkaline in nature — as has 
been done with great success since FiscHER’s experiments, 
is also based upon a colloid-chemical understanding of their 
effects. Such salts inhibit the swelling and coagulating 
effects of the acids formed in the pathologically affected 
tissues just as they decrease their effects upon simple mix- 
tures of colloids in a test tube. 

I can only mention in passing that inflammation has 
also been discussed from a colloid-chemical point of view,? 
that the unknown substance which is associated with 
goiter? is undoubtedly colloid in nature, that the immune 
reactions in their mutual adsorptions and _ precipitations 
offer a field full of unlimited possibilities for the appli- 
cations of colloid chemistry, that the so-called ‘‘colloid 
reactions”? of serum or cerebrospinal fluid with inorganic 
colloids of gold (LANGnh’s reaction), berlin blue or mastic 
are indicative of the diagnostic applications which may be 
made of colloid chemistry, that promising beginnings of 


( 
1 See Martin H. Fiscner, (idema and N ephritis, third edition, 648, New 
York, 1921; as well as Kolloidchem. Beihefte, 4, 343 (1913). 
2 A. Oswa Lp, Zeitschr. f. exp. Pathol. u. Therapie, 8, 226 (1910); a review 
is found in Koll.-Zeitschr., 9, 251 (1911). 
’ See E. Brrcuur, Ergebnisse der Chirurgie u. Orthopidie, 5, 133; Zeitschr. 
f. exper. Pathol., 9, etc. 


188 COLLOID CHEMISTRY 


a colloid-chemical theory of narcosis have been made.! 
Furthermore, the colloid state of bile salts and of uric acid 
is of importance in unravelling the origin of gallstones and 
the concretions found in gout, while protein solutions: are 
able to keep these materials in solution in concentrations 
much beyond their solubility in water (H. ScuapE, Wo. 
Pavuut, H. Becuuoup); an adsorption therapy has de- 
veloped in which various diseases are treated with adsorb- 
ents like charcoal, different earths, etc.; and colloid-chem- 
ical points of view have brought light into the nature of some 
of the processes observed in the healing of wounds.” You 
observe that one needs much breath merely to list the chap- 
ters in which colloid chemistry has already proved itself 
of service in medicine. 

I would also point out that a whole series of inorganic 
colloids is now being used therapeutically — colloid sul- 
phur in skin diseases, colloid mercury and mercurial salts 
in syphilis, colloid nickel in meningitis, colloid silver for 
the antiseptic treatment of wounds and in the manage- 
ment of infectious diseases like gonorrhea and ophthalmia. 
In ophthalmology colloid silver has almost completely dis- 
placed silver nitrate. It is of much interest that in com- 
parative studies of the disinfecting action of different 
disinfectants colloid preparations of mercury have proved 
themselves of extraordinary activity even when used on 
many different types of bacteria. Colloid preparations of 
mercury according to the recent studies of H. FRIEDENTHAL 
seem to be the most powerful chemical disinfectants that we 
now possess. But even the well known and important 
salvarsan is, in aqueous solution, in a colloid state, as I have 
long known from ultramicroscopic study and as has been 

1 See S. Lorwe, Biochem. Zeitschr., 57, 161 (1913), where references to 
the literature may be found. 

2 W. von Gaza, Koll.-Zeitschr., 23, 1 (1918). 

3 See G. SropEt, Les colloides en Biologie et en Therapeutique, Paris, 
1908; a review appears in Koll.-Zeitschr., 4, 321 (1909); H. FrrepDENTHAL, 


Biochem. Zeitschr., 94, 47 (1919); R. Epmr, Schweiz. Apoth. Ztg., Nos. 
29-33 (1918) (a review lecture). 


SCIENTIFIC APPLICATIONS 189 


proved more recently by others... Some years ago the in- 
teresting therapeutic applications of an inorganic sol, 
palladium hydroxid, were much discussed. It was used to 
combat obesity. Treatment consisted of the hypodermatic 
injection of the material into the fatty areas to be reduced, 
the injection being associated, it was said, not only with 
no unpleasant sensations but actually pleasant ones.? 
We have not heard of further progress along these lines but 
this is to be attributed, perhaps, to the war years which 
did away so largely with the material upon which the ex- 
periments could be made. 

Of course — and I would like to emphasize this point in 
conclusion — the pendulum has perhaps swung too far in 
this application of colloid-chemical points of view to prob- 
lems in materia medica and therapy. The advantage of 
colloid pharmaceutical preparations over others has perhaps 
been stressed too heavily; in England, for example, the 
claims have been carried to a point which not only seems 
nonsensical but one which must embitter any honest, no 
matter how enthusiastic, colloid chemist. Nor in the 
matter of theory does one need to go as far as J. TRAUBE 
who represents perhaps the most extreme of the modern 
appliers of colloid-chemical points of view to practice.’ 
But that aside from its chemical constitution, colloid-chem- 
ical factors like degree of dispersion and degree of solvation 
play an important part in the action of any pharmaceutical 
preparation — this is a view shared by even the most con- 
servative workers in these fields and one to which, in good 
conscience, I too hold. 


1 H. Bauer, Arb. Inst. f. exper. Therapie, Frankfurt, No. 8 (1919). See also 
A. Brnz and collaborators, Ber. d. Dtsch. chem. Ges., 53, 416 (1920). 

2 See M. Kaurrmann, Miinchener Medizin. Wochenschr., 525 (19138). 

3 The numerous papers of J. TRAUBE are widely scattered. References to 
them and reviews may be found in the indices of the Kolloid-Zeitschrift; a 
recent paper dealing chiefly with the colloid chemistry of pharmaceutical 
preparations may be found in Biochem. Zeitschr., 98, 177 (1919). 


190 COLLOID CHEMISTRY 


$27. 


With this I must end the list of the applications which 
have been made of colloid chemistry to neighboring sciences. 
I do it with the hope that I may have convinced you of 
the inadequacy of any such lecture as today’s to portray 
the countless applications that have thus been made or 
can still be made. I know that every one of you could, 
from the special fields of your particular endeavors, at 
once state a problem which might be investigated from a 
colloid-chemical point of view and which I have not at 
all touched upon. The day seems already here when no 
one speaker can by himself get even an approximately 
complete view of the whole field comprised under this 
heading of the applications of colloid chemistry to science. 


V 


SOME TECHNICAL APPLICATIONS OF 
COLLOID CHEMISTRY. 





FIFTH LECTURE. 


SOME TECHNICAL APPLICATIONS OF 
COLLOID CHEMISTRY. 


I sHAuL in this last lecture survey the applications of 
colloid chemistry to some technical, industrial and prac- 
tical problems. You might begin by asking whether there 
1s any purpose in spending a whole hour in discussing 
these things. The concept of the colloids has become a 
familiar one only recently and so it might be concluded 
that the teachings of the colloid chemist find application 
in these practical fields only along narrow and specialized 
lines. Have enough and sufficiently important applica- 
tions of colloid chemistry to technology really been made 
to justify spending a whole hour upon the subject? Let 
me in answer ask you to accompany me for a moment. 


§1. 


The clothes you wear, be they wool, cotton or silk, are 
animal or plant gels. They are dyed with colors which, in 
many instances, as the indigos and the blacks, are colloid 
in type. In the process of dyeing, adsorption and other 
colloid-chemical reactions take place between the colloid 
substrates of the fabric and the colloid dyes which color 
them. The leather of your shoes is an animal gel, closely 
related in its general properties to that prototype of the 
colloids, gelatin. Leather is tanned with substances of 
which the majority are colloids, and the whole process of 
tanning is punctuated with the colloid phenomena of hy- 
dration, dehydration and adsorption. The wood of the 


chairs in which you rest is made of cellulose, which in all 
193 


194 COLLOID CHEMISTRY 


its various forms is colloid in nature. The colloid swelling 
of wood, as I emphasized earlier, was used by the old 
Egyptians to aid their quarrying of stone. The woods 
of your chairs are held together by glue or with metals. 
You already know glue to be a colloid, but it may surprise 
you to learn that colloid chemistry has much to say in 
metallurgy and that steel, for instance, is a colloid solid 
solution. We shall return to this question. The paper 
upon which you write is essentially cellulose, in other 
words, again colloid. It has been given a body by being 
mixed with water-glass, with rosin or some similar material, 
in other words, with various colloids. The ink in your 
fountain pens is probably also colloid if it is the ordinary 
iron tannate, and colloid, too, is the hard rubber of your 
pen holders, prepared from that notoriously colloid mother 
substance, soft rubber. ; 

I could continue this list indefinitely, pointing in this 
manner to one colloid after another in your immediate 
surroundings and belonging to the things of your every- 
day life. Perhaps you think — perhaps since yesterday’s 
lecture you think you know — that I am possessed of a 
colloid mania because I see colloids everywhere. Let me 
admit that I do see colloids everywhere, only I do not 
believe that because of this I must be adjudged insane. 
It is simply a fact that colloids constitute the most universal 
and the commonest of all the things we know. We need only 
to look at the sky, at the earth, or at ourselves to dis- 
cover colloids or substances closely allied to them. We 
begin the day with a colloid practice — that of washing — 
and we may end it with one in a bedtime drink of colloid 
tea or coffee. Even if you make it beer, you still consume 
colloid. JI make these remarks in full earnest and with the 
request that if I do not prove my assertions to your satis- 
faction, you challenge me in the matter. 

These facts leave no doubts in our minds as to the wealth 
and variety of the possible technical and practical appli- 
cations of colloid chemistry. We only become conscious 


TECHNICAL APPLICATIONS 195 


again of a great difficulty in making a proper choice of 
illustrations from the wealth of material before us. 

To this difficulty come two others. Colloid chemistry 
as a systematically studied science is still very young. It 
cannot therefore be expected that any conscious applica- 
tion of colloid chemistry to technology has as yet been made 
in anything like the degree possible or probable. Many. 
technical experts do not as yet even know that in their 
every-day practices they are working in colloids and that 
they should, in consequence, employ the fruits of scientific 
colloid chemistry in their various endeavors. This fact 
is often brought home to the colloid chemist who enters 
into discussion with practical men — something which, by 
the way, every scientist should do as often as possible. 

I remember a conversation with a brick manufacturer 
who complained because two lots of clay which were alike 
chemically, yielded products of very different qualities. 
I expressed the conviction that a difference in the colloid 
state of the clays was probably responsible. ‘Colloid 
state, what do you mean by that?” he answered. Our 
conversation then turned to a discussion of colloids and he 
thus heard for the first time of the fundamental properties 
of materials in which he had worked for decades. Need- 
less to add he became an enthusiast in colloid chemistry 
and I doubt not that he is now a regular subscriber to the 
Kolloid-Zertschrift. 

What I have said of the brick manufacturer is true of 
many other branches of industry. In many of them even 
that first of steps needs yet to be taken, namely, that of 
recognizing the colloid nature of the materials being worked 
upon and the colloid nature of the processes being used in 
manufacture. The details of technical procedure need to 
be rewritten in the terms of colloid chemistry. Let it be 
clearly understood that this does not mean a mere restate- 
ment of facts and problems in different terms. When I 
say that rubber or cellulose is a solid colloid or that this 
or that technical process represents an adsorption phe- 


196 COLLOID CHEMISTRY 


nomenon, I have to assume full responsibility for such 
statements and must be able to prove that the substances 
concerned show the fundamental properties of the colloid 
state or that the processes declared adsorptive in nature 
obey the adsorption laws. Such new definition in the 
terms of colloid chemistry is by no means always as simple 
as might at first appear. I beg you to remember this 
when in the following discussion I am often merely able 
to state that this is a technically interesting colloid as 
shown by such and such properties; that these processes 
are probably colloid-chemical in nature, ete. Complete 
colloid-chemical analysis and an accurate distinction be- 
tween such processes in technological practice as are colloid- 
chemical in nature and such as are not has been found 
possible thus far in only isolated instances. There is a 
wealth of work to be done here, interesting not only from 
a scientific standpoint but from a practical one as well. 


§2. 


Permit me after this lengthy introduction to enter at 
once upon a consideration of the use of the inorganic col- 
loids in practice. The whole list of elements finds use in 
colloid form in industry; to a few of these elements I would 
like to call your attention. 

An interesting and characteristically American product 
consisting of an element in colloid form is the so-called 
ACHESON graphite.1. I have shown you this before under 
the name aquadag as a dispersion in an aquéous dispersion 
medium. I show it to you again dispersed in a mineral 
oil under the name otldag. These two preparations, which 
are much used as lubricants, prove on investigation to 
be typical colloids. This fact is revealed by ultramicro- 
scopic examination, by the migration of the black phase 
in the electric field, by the precipitation effects produced 
through the addition of acids or sodium chlorid, ete. This 


1 I am greatly indebted to Dr. AcHzEson for a large quantity of demon- 
stration material. . 


TECHNICAL APPLICATIONS 197 


technically important material serves also to illustrate my 
remark that much work is done with colloids without the 
workers being conscious that they are working with this 
type of material. AcHEsoN did not begin with intent to 
prepare colloid graphite as is evident from his interesting 
addresses upon the history of his discovery. He only 
afterwards became conscious of the similarity and the 
relationships of his preparation to colloid types of materials. 
Aside from the fact that aquadag, because of its black 
color and accessibility is useful as material upon which to 
demonstrate the fundamental properties of colloids (diffu- 
sion, dialysis, filtration, electrophoresis, coagulation, ad-— 
sorption, etc.) it is a preparation which is of great colloid- 
chemical interest in other directions as well. First of all, 
its method of preparation is interesting, of which ACHESON 
says that he finds it described in the Bible, or rather in 
the reports contained in the Bible of the methods employed 
by the Egyptians in the production of high-quality bricks. 
The Egyptians used straw infusions and other liquids rich 
in tannin in order to obtain uniform and finely divided 
clay. Inthe same fashion ACHESON made use of commercial 
tannin preparations in order to obtain a highly dispersed, 
stabile and concentrated preparation of ground graphite. 
Just as when colloid gold is made with tannin, so in the 
preparation of colloid clays and of graphite, the tannins act 
as typical protective colloids. When graphite is ground 
in the presence of tannin the highly dispersed and colloid 
particles of graphite are encompassed as formed by the 
strongly hydrated tannin and so prevented from running 
together again into larger aggregates. But because of the 
great stability of the tannin toward electrolytes, it now 
becomes possible to evaporate a part of the dispersion 
medium “until a graphite paste is obtained without the 
graphite settling out. The presence of this protective 
colloid also helps to keep the graphite finely divided when 
the lubricant in practice is exposed to the precipitation 
dangers incident to coming in contact with electrolytes. 


198: COLLOID CHEMISTRY 


Another point of interest is that the lubricating action 
of the graphite is intimately connected with its degree of 
dispersion. Even ordinary, coarsely dispersed graphite is 
a good lubricant, but its effectiveness is much increased 
by simple grinding, when the so-called gredag is obtained. 
But these coarse preparations are excelled by colloid graph- 
ite. We see here again a property, namely, lubricating 
activity, which increases steadily with increase in degree 
of dispersion. As the technologists among you well recog- 
nize, we still know very little regarding the properties 
which make a substance a good lubricant. Perhaps a 
closer study of this relationship between lubricating effect 
and degree of dispersion in heterogeneous lubricating ma- 
terials may bring us some light, which may in its turn 
tell us what makes certain homogeneous liquids good 
lubricants and others not. 


§3. 


Other elements which are used in colloid form in techno- 
logical processes are seen in the metals. An interesting 
and old application of colloid chemistry to technology is 
seen in the use of colloid metals for the production of 
encandescent light filaments. As you know, the lighting 
expert is constantly trying to bring the various light- 
giving bodies used in his lamps to as high a temperature 
as possible, since by this means he shifts the relation of 
the visible to the invisible, or heat rays toward the side 
of the light rays. For this reason the attempt is old which 
would replace the easily vaporized carbon filaments of our 
older lamps by the less volatile metallic filaments of tung- 
sten, tantalum, etc. These metals have, however, the 
unwelcome property of great brittleness so that they can 
. not be drawn into threads. To overcome the difficulty 
the metals were used in finely divided form. Colloid 
powders of the metals were made into pastes, often through 
the addition of some hydratable organic colloid, and the 
pastes were then squeezed through fine openings (as in 


TECHNICAL APPLICATIONS 199 


the manufacture of artificial silk), and in this way very 
fine threads of the metals were obtained. I am able to 
show you here some hair-like threads of tungsten prepared 
in this manner (demonstration).1 

Of special interest is the method of Kuzru for the prep- 
aration of such colloid metals in powder or paste form. 
Prolonged grinding alone suffices to convert a low percent 
of the metal into the form of the colloidally divided powder. 
I show you some tungsten powder prepared in this way. 
Thrown upon a filter you observe that it runs through 
as soon as some distilled water is poured upon it (demon- 
stration). This mechanical grinding takes much time and 
is very expensive. It is simpler and cheaper to make a 
colloid mud of the metal by the method of Kuzren. In 
this the powdered metal is successively and repeatedly 
treated with acids and alkalies interspersed with washings 
in distilled water. The theory of the process is about 
as follows. 

In a dilute acid the surface layer of the coarsely dispersed 
particles of metal goes into solution in finer form, yielding 
a smaller grain. This reduces a part of the powder to 
colloid dimensions. If the acid were allowed to act too 
long, the colloid thus formed would dissolve completely 
and disappear. The acid must therefore be neutralized 
and washed away, while the colloid which has been formed 
is separated by precipitation or filtration from the still 
coarsely divided material. By repeating the process, an- 
other part of the metal is brought into the colloid state, 
the whole procedure being repeated time after time until 
all the metal is gotten into colloid form. 

We have however learned recently how to alloy the metals 
we are discussing with others and how through heat man- 
ipulation to get them into ductile form. As progress has 
been made along these lines, the more cumbersome colloid 
method discussed above has been displaced. 


1 Colloid incandescent lamp filaments were kindly placed at my disposal 
by the Chemische Fabrik von HEYDEN. | 


200 y COLLOID CHEMISTRY 


§4. 


Colloid metals and colloid metallic compounds are much 
used to give color to various materials. Ruby glass owes its 
red color to the presence of colloid gold. I show you three 
specimens which are ‘‘solid solutions” of gold in three 
very different and characteristic degrees of dispersion 
(demonstration).!. The first is an almost clear and but 
slightly yellow mass of glass. This is obtained immedi- 
ately after dissolving the solid gold salt in the glass. ‘There 
is obtained in this way a molecularly dispersed solution 
of the gold in the glass, and one which, in consequence, 
is ultramicroscopically empty. The second preparation is 
the ordinary ruby glass in which the gold is contained in 
a colloid state. The third specimen is deep blue by trans- 
mitted light and orange brown by reflected light. The 
specimen is also distinctly turbid. It springs from a failure 
in glass manufacture in that, presumably through a too long 
heating of the glass, a coagulation of the red gold particles 
to the more coarsely dispersed blue particles has taken place 
—just such a change as I showed you in an aqueous dis- 
persion medium when I coagulated the red gold (produced 
through reduction of gold chlorid by tannin) to blue gold 
through the addition of acid. These same facts as illus- 
trated in the case of glass prove of what little importance 
is the kind of dispersion medium and how much depends 
upon the degree of dispersion in determining the varia- 
tions in color in this substance. 

Silver colloidally dispersed in glass makes it yellow or 
brown. Selenium colors it beautifully red or violet. The 
colloid metallic hydroxids also impart to glass some beauti- 
ful colors, as illustrated in the production artificially of 
the precious stones. The artificial rubies owe their color 
’ presumably to a colloid chromium, as do the artificial 
alexandrites, ete. 


1 Different specimens of gold ruby glass were kindly placed at my dis- 
posal by Popper AnD Sons of New York, 


TECHNICAL APPLICATIONS 201 


85. 


An interesting illustration of color due to a colloid ele- 
ment is seen in the case of ultramarine. There still rages 
an old debate concerning the causes for the color in this 
mixture of different silicates, borates, etc., with sulphur 
or sulphur compounds. Even recently, nothing short of 
desperate efforts have been made to explain the color of 
this dye substance on the basis of its ‘“‘chemical constitu- 
tion.” I say desperate because not only are the quantita- 
tive relationships found in the different ultramarines totally 
different, but on heating the normal ultramarine, uncolored, 
grey, yellow, red, blue and even green ultramarines can be 
produced, as shown in these specimens (demonstration). 
On the basis of differences in chemical constitution we 
would have to assume that each of these different colors 
represented a different chemical compound. 

What we observe is entirely analogous to what we dis- 
cussed previously when dealing with the photohaloids. 
We can produce blue and green solutions of sulphur by 
simply introducing this element into molten sodium chlo- 
rid, into a borax bead, into liquid ammonia, into hot or- 
ganic liquids like glycerin or by simple reduction of a thio- 
sulphate with phosphoric acid as I showed you in the sec- 
ond lecture. These facts render it most improbable that 
ultramarine is blue because of the existence in it of a specific 
blue sulphur compound. We have therefore come to the 
conclusion that ultramarine represents a solid solution of 
highly dispersed sulphur. The degree of dispersion may 
oscillate between molecular and colloid dimensions,? and 
as this happens the different colors discussed above which 

1 See Kolloidchem. Beih., 2, 449 (1911); also P. P. von Wermarn, Koll.- 
Zeitschr., 20, 278. (1917); R. Aversacn, Koll.-Zeitschr., 27, 223 (1920). 

2 The assumption that we have to do with a solid solution of elementary 
sulphur in the case of the ultramarines was first made by J. HorrMANN [see, 
for example, Koll.-Zeitschr., 10, 275 (1912)], but that we deal with solid 
solutions possessed of a colloid degree of dispersion or of one approximating 


these dimensions, seems first to have been expressed by me [Kolloidchem. 
Beih., 2, 449 (1911); also Koll.-Zeitschr., 12, 61 (1913)]. 


202 COLLOID CHEMISTRY 


represent different degrees in the dispersion of the element 
sulphur are produced. 

Of the many facts which confirm this view, I would like 
to emphasize the analogy between the production of ultra- 
marine and the production of ruby glass, of blue rock salt, 
etc. In making ultramarine the necessary salts and the 
sulphur are melted together at a high temperature. This 
yields the greyish white or yellowish ‘‘mother of ultra- 
marine.” This product is then reheated, cooled and re- 
heated again, just as in the case of ruby glass, until the 
requisite color is obtained. The original product is ob- 
viously a molecularly dispersed solution, the particles of 
which, through reheating, are permitted to condense to 
colloid dimensions. Support for the correctness of this 
view may be found in mineralogy. Mineralogists are 
familiar with a complex, sulphur-rich, silicate compound 
known as hauynite which appears in different colors rang- 
ing from colorless to green and blue. It has been shown! 
that the colorless varieties may be colored blue or green 
by heating them with sulphur in a closed tube, an experi- 
ment entirely analogous to the production of blue rock 
salt by heating this with metallic sodium. 

There exist reasons for believing that in the colors of 
many of the so-called sulphur dyestuffs (dyes produced by 
melting together sulphur and different organic compounds 
in the presence of alkali?) we have also to do with similar 
solid solutions of highly dispersed sulphur. There cer- 
tainly exists little hope of explaining their colors on the 
basis of chemical constitution. 

Let me in passing emphasize that the alkali used in the 
preparation of either the inorganic or the organic sulphur 


1 See Koll.-Zeitschr., 12, 62 (1913); also N AUMANN-ZIRKEL, Lehrbuch 
der Mineralogie, 665. 

2 See O. Lanan, Die Schwefelfarbstoffe, ihre Herstellung usw., Leipzig, 
1912, as well as the ‘review of this volume in the Koll.-Zeitschr., cited in the 
previous footnote. See also W. ZAnxur, Zeitschr. f. angew. Chem., 32, 49, 
(1919). 


TECHNICAL APPLICATIONS 203 


systems tends to increase their degrees of dispersion. It 
has, in other words, a peptizing or stabilizing influence. 

As you doubtless know already, many of the native gels 
are used directly as coloring substances. I need but men- 
tion the hydrated or burnt iron hydroxid gels (terra di 
siena, umber, yellow and red ochre, etc.). 


$6. 


Colloid chemistry finds many applications in photography 
and the various graphic arts. I have already touched upon 
the fact that many inks like the old iron tannate inks and 
india ink are colloid solutions. Printing inks are given 
their proper body by being mixed with colloids. Gelatin 
and other colloid mixtures are used in different kinds of 
color printing, etc. Not only does the colloid chemistry 
of the photohaloids play a great réle in photography, but 
many other colloid-chemical processes are encountered in 
the manufacture of dry plates. This is illustrated in the 
“‘ripening”’ of the photosensitive emulsions until an opti- 
mal size of ‘‘granule”’ is obtained, and in the different 
methods employed for the developing, intensifying and 
printing of dry plates. To those of you who are interested 
in these subjects I suggest a perusal of the many contri- 
butions of Luppo-Cramer, R. E. Lirsecane and others.! 


87. 


Of other fields in which colloid chemistry plays an im- 
portant part I would emphasize those of ceramics and the 
hydraulic cements. The earths and clays are in great 
measure typical gels, consisting, as they do, so largely of 
aluminium silicate and iron hydroxid, admixed with or- 
ganic colloids of the type of the humus acids. The plas- 
ticity of the ceramic clays is dependent in large part upon 
their colloid content; and their changes with time, the 
effects of added straw infusions, of treatment with ammonia, 


1 See the literature cited in the first footnote on page 140. 


204 COLLOID CHEMISTRY 


etc., are all processes intended either to increase their ab- 
solute colloid content or to increase the peptization or 
hydration of this content. 

The effect of alkalies upon ceramic clays is so great and 
plays so important a rdle in modern industry that it ‘‘has 
produced a revolution in ceramics,” as one of the first 
authorities in this field has expressed it. Addition of 
alkalies tends to “‘liquefy” the clays. A stiff clay when 
treated with proper amounts of alkali loses its stiffness 
and changes to a liquid. On the other hand, dry clay 
mixed from the beginning with relatively small amounts 
of water, but containing some alkali, yields a fluid or semi- 
fluid mass. The technical importance of these findings 
resides in the fact that by such means much smaller amounts 
of water need to be used and so a faster drying of the clay 
moulds is obtained preparatory to firing. But in this 
fashion the much-feared cracking and deformation ordinarily 
incident to the first drying is also greatly reduced. These 
alkalinized clays can, moreover, be poured and in this 
manner large objects like bathtubs can be made much 
more easily than by the older moulding process.! 

The explanation of how the alkali produces its effects 
is somewhat complicated; in it at least three to four differ- 
ent colloid-chemical processes overlap each other. The 
clay, an electro-negative colloid, is peptized by the addi- 
tion of small but definite amounts of alkali. This puts 
it into a more highly dispersed state, an effect entirely 
similar to that observed when other negative colloids, like 
those of the metals, are treated in this manner. | Second, 
a swelling is produced in the particles of aluminium sili- 
cate, a process which apparently attains its optimum at 

? A discussion of the literature and of the patents covering the use of 
alkalies in ceramics may be found in J. K. NEUBERT, Kolloidchem. Beih., 
4, 261 (1913). A review of modern advances in ceramics with special Soren. 
ence to colloid chemistry may be found in H. ArNoxp, Chem. Ztg., 413, 426, 
439 (1918). The important paper of E. Popszus, Koll. -Zeitschr., 20, 65 


(1917) where references to his earlier studies may be found Te special 
emphasis. 


TECHNICAL APPLICATIONS 205 


a somewhat higher concentration of the alkali than that 
_ needed for peptization. To bring about this swelling effect, 
time is needed. Third, the alkali affects the organic col- 
loids like tannin, humus acids, etc., constantly present 
under industrial circumstances. These substances are also 
peptized in low concentrations of the alkali, while larger 
amounts not only leach them out, but bring them into a 
molecularly dispersed condition. In this way they lose 
their importance as ‘‘ protective”? hydrated emulsoids. The 
three effects work side by side, so that when the element 
of time is also added a fairly complicated picture results. 
In short periods of time, peptization and increase in vis- 
cosity tend to nullify each other. It is usually held that 
the clays which on admixture with water take longest to 
settle out are also the most fluid, but this parallelism which 
is much used analytically may be lost as with more time 
the effect of the alkali brings about an increased swelling 
of the clay particles.! 


88. 


The technically important process of setting, as observed 
in the hydraulic cements (cement and mortar), may be 
defined chemically as a reaction between calcium and silicic 
acid associated with a taking up of considerable amounts of 
water. These are the fundamental changes which occur in 
the setting of all the cements even though the materials that 
are mixed with the cements and mortars are for various 
practical purposes very different. 

These fundamental changes of chemical combination ote 
hydration may already be observed when calcium oxid and 
sand are mixed together as in ordinary mortar. In the case 
of Portland cement, the effects of admixture with calcium 
and iron hydroxid are superadded. It is not our problem 
to say how much justice there is in the various specific 
chemical assumptions which have been made to explain the 


1 See the detailed discussion by Jon. K. Nevuserrt cited in the previous 
footnote. 


206 COLLOID CHEMISTRY 


nature of these fundamental setting processes. It is widely 
believed that there comes into play a whole series of different 
and, in part, crystalline compounds (mono-, di-, tri-caleium 
silicate; alite, belite, celite, etc.). I would only emphasize 
that every exclusively chemical theory is unable to explain 
the physical accompaniments which are so characteristic of 
the setting process. Chemical reactions associated with 
hydration, like those observed in the mortars and cements, 
take place between many substances without the reaction 
mixture developing the characteristic physical properties 
observed in the mortars and cements. In the setting of 
mortar or cement there must take place certain special 
changes besides the chemical which are responsible for the 
physical peculiarities resulting from these reactions. Recent 
investigations have proved the existence of such and it has 
been found that we again deal with colloid-chemical proc- 
esses. 

From microscopic study of the setting process, it was long 
known that when cement is mixed with water, numerous 
needle-like crystals begin to form around every cement 
particle, consisting, supposedly, of calcium mono-silicate, 
while larger and smaller hexagonal crystals, presumably 
tri-calcium aluminate and calcium hydrate, appear in the 
interstices. But there appears also a structure which, while 
noted before, has had a proper significance given it only 
recently. There forms about every cement particle a gel of 
calcium silicate, the volume of which increases steadily during 
the pracess of setting until it fills not only the interstices between 
the crystalline needles but all those between the individual 
cement particles. It is the formation of this gel which is 
undoubtedly the most important factor in the process of 
setting and which gives the solid cement its specific physical 
properties. The gel serves to bind together not only the 


* See especially W. Micuaztis, Koll.-Zeitschr., 5, 9 (1909); 7, 320 (1910) ; 
S. KerserMann, Kolloidchem. Beih., 1, 423 (1910), where references to the 
literature may be found; as well as numerous shorter papers by P. RoHLAND 
to be found in the Kolloid-Zeitschrift. 


TECHNICAL APPLICATIONS 207 


individual crystals surrounding a given cement particle, but 
also the crystals of neighboring particles. Cement particles 
with their crystals become imbedded in a common sheath 
of gelatinous substance (Fig. 45) and it is this fact which 





Fie. 45. — Diagram illustrating the changes incident to the setting of cement 
according to W. Micuar is. The serrated lines indicate the outlines of 
the gel. 


gives cement its solidity. Felting of the crystalline needles 
could by itself not explain this solidity. The cement tends 
to become progressively harder as more and more water is 
taken from the binding gel into the innermost layers of the 
cement particles themselves. The gel which we have been 
discussing can be stained by various dyes (like anthrapur- 
purin) and so be made to stand out from the rest of the 
cement. Perhaps you would like to see an ultramicroscopic 
picture of this setting process. I show you such in Fig. 
46, in which you may see very distinctly the needle-like 
crystals protruding from the gel mass. 


208 COLLOID CHEMISTRY 


If we accept the production of such a gel as the char- 
acteristic element in the process of setting, as has been 
especially well insisted upon by W. MICHAELIS, we at once 
obtain the explanation of a whole series of technological 





Fig. 46. — Ultramicroscopic photograph of cement in the process of setting, 
according to H. AmMBronn. Observe the delicate crystalline needles pro- 
truding from the gel envelopes. 


details. In order that the gel formation may be complete, 
there must, of course, be enough water present. It is for 
this reason that in testing for the maximal rigidity of cement 
samples, it is necessary to allow these to harden under water. 


TECHNICAL APPLICATIONS 209 


Of especial technical importance is a regulation of ‘the rate 
of the setting. Setting is retarded, for example, through 
the addition of such hydrophilic organic colloids as glue. 
This is explained colloid-chemically by finding that the glue 
takes up the water and then gradually yields the water to 
the silicate gel as this forms. Conversely, the setting 
process is hastened by adding organic acids like acetic acid. 
Colloid-chemically this means that acetic acid favors the 
formation of the silicate gel. 

Entirely in agreement with this colloid-chemical theory of 
setting is the fact that so simple a process as the hydration 
of plaster-of-paris is also associated with colloid-chemical 
changes, according to the observations of A. Cavazzi, 
J. TRAuBE, P. Wotsxi1 and myself.!. A 3 to 4 percent sus- 
pension of plaster-of-paris shows, for example, a beautiful 
and regular increase in its viscosity with time; the process 
of setting can, in other words, be followed quantitatively 
in this way,as can the gelation of a typical emulsion colloid. 
Electrolytes favor or retard the process according to the 
HoFMEISTER ion series; while the addition of 2 percent 
glue retards the setting so markedly that there is no longer 
any change in the viscosity of the suspension with time. 


89. 

I come next to an especially important chapter in tech- 
nology, that of the application of colloid chemistry to 
metallurgy. I must preface my remarks by saying that the 
possibilities which colloid chemistry holds for the explanation 
of many metallurgical problems have hardly, as yet, been 
recognized. If in the following I venture some colloid- 
chemical points of view to explain various metallurgical 
questions and you find these not touched upon in your 
studies of the orthodox authorities who work in these fields, 
please know that I do this only because I am convinced that 


1 Woutraanc Ostwatp and P. Wotsxt, Koll.-Zeitschr., 27, 78 (1920) 
where references to the older literature may be found. 


210 COLLOID CHEMISTRY 


the colloid-chemical or dispersoid-chemical point of view is 
going to have a remarkable future in metallurgy. 

Problems which are definitely colloid-chemical in nature 
appear in the initial processes of mining. It is well known, 
for example, that gold can only with great difficulty be 
extracted from deposits which are rich in various earths or 
clays. The metal present in these materials is probably 
highly dispersed; but, more than this, it is probably held so 
fast or is so surrounded by the gelatinous hydroxids and 
silicates of aluminium and iron constituting these earths 
that the ordinary washing schemes do not suffice to extract 
the gold. The gold is ‘‘masked”’ by the hydrated colloids 
just as iron is masked through the presence of organic sub- 
stances — in other words, the ordinary analytical reactions 
of the metal are not obtained until the organic parts have 
been destroyed. In order to make use of these minerals, it 
would be necessary to destroy the various inorganic colloids 
or to separate the metal from its adsorption complexes — a 
colloid-chemical problem which up to the present time has not 
been solved satisfactorily. Even relatively pure gold when 
colloidally dispersed is not taken up easily when shaken with 
mercury — a fact no doubt attributable to the difficulties 
incident to obtaining adequate contact between the colloid 
particles and the surface of the mercury. 

The practice of flotation has recently assumed extraor- 
dinary prominence in mining. It will be simplest, perhaps, 
if I demonstrate an experiment in flotation in order to fa- - 
miliarize you with the fundamental principles which are 
at stake. I have in this mortar a mixture of white kaolin 
and some finely ground carbon. I could use graphite in 
place of the latter. When this mixture, which is to represent 
a ground-up ore, is shaken with water a grayish-black sus- 
pension is obtained in which the carbon or the graphite 
represents a valuable ‘‘ore’”’ mixed in unpleasant and in- 
timate fashion with the valueless kaolin representative of 
the ‘‘gangue.”’ The technical problem at stake is this: 
how can we get the carbon or the graphite free from the 


TECHNICAL APPLICATIONS vAG 


kaolin? This end can be accomplished in simple and clean- 
cut fashion as follows. I add to the flask containing the 
gray suspension a few cubic centimeters of xylol, benzol or 
some other hydrocarbon not miscible with water — even 
kerosene will do — and shake a minute or two. An emul- 
sion is formed but this soon creams and the hydrocarbon 
droplets run together. At the same time, however, — and 
this is the scientifically and technically interesting moment 
— the hydrocarbon droplets take to the top with them the carbon 
or the graphite particles but not those of kaolin. You see for 
yourselves how the carbon particles. have formed a black 
layer at the top while the slightly gray kaolin, the ‘‘gangue,”’ 
has settled to the bottom.! The separation we sought to 
accomplish is therefore made. Sulphid ores can in this 
fashion be separated from their accompanying gangue. 
As a matter of fact, ground up ores are often so treated as 
to convert their contained metals into sulphids in order 
that by flotation means they may be concentrated the more 
easily. 
_ What now is the theory of this and similar technological 
procedures? I can answer this best by showing you another 
experiment. I have here six pieces of paper, each of an 
area of two to four square centimeters, cut from the ad- 
vertising pages of an illustrated journal. Three of the 
pieces are white on both sides, that is to say, nothing is 
printed upon them. The other three have been printed 
upon on one side. I shake up the six leaflets in water until 
they have been freed from adhering air and have sunk to 
the bottom of the Erlenmeyer flask (demonstration). 
A few cubic centimeters of xylol are now added and the 
flask is again shaken for a moment. The coarse emulsion 
which forms, separates rapidly. The paper leaflets are now, 
however, no longer evenly distributed. The three white 
papers have sunk in the water but the three black ones 
1 Details regarding this and other experiments in flotation may be found 


in Wotreane Ostwa.p, Kleines Praktikum der Kolloidchemie, 110, Dresden, 
1920. 


212 COLLOID CHEMISTRY 


float high in the surface between the water and the xylol 
with their black faces turned toward the xylol.1 Why? 
Evidently because printer’s ink ground up in oil 1s more 
casily wetted by xylol than by water and it takes work to 
tear the layer, printer’s ink-xylol, in two. ‘This is a meas- 
ure of the force with which the paper adheres to the xylol. 
In similar fashion, carbon, graphite, sulphids, etc., are 
better wetted by hydrocarbons than by water wherefore 
they too ‘‘stick” better to the droplets of the former than 
to the latter and so rise with the hydrocarbon droplets in 
the ‘‘flotation” treatment of ores. This is, in brief, the 
explanation of its fundamental nature. 

Another phenomenon in metallurgy, colloid-chemical in 
nature, is seen in the deposition of metals by electrolytic 
means. The structure of an electrolytically produced pre- 
cipitate is markedly influenced, for instance, through the 
addition of traces of organic colloids like gelatin, albumin or 
dextrin. When present in certain definite concentrations, 
these colloids bring about a great increase in the degree of 
dispersion of the electrolytically produced precipitate. In- 
stead of voluminous macro-crystalline or micro-crystalline 
precipitates, there are obtained dense, finely structured 
layers showing a smooth polish. The Germans call this 
‘Glanzgalvanisation.’? Colloid-chemically we may under- 
stand what happens by remembering that the added colloids 
are of the group of the hydrated emulsoids. As the pres- — 
ence of tannin or gelatin tends to produce gold in highly dis- 
persed form, just so do the hydrated emulsoids act in the case 
of the electrolytic deposition of the metals — a process which 
also represents a condensation of highly dispersed particles 
to grosser ones. I can only mention in passing that the’ 


. 1 The experiment goes well provided print paper which has been sized (with 
glue) is used. If ordinary unsized newsprint is employed, indefinite results 
are obtained because the oil from the printer’s ink has soaked into all the 
paper and usually penetrated it. 

2 See, for example, E. Mizumr, Zeitschr. f. Elektrochemie, 317 (1906) ; 
a review is found in Koll.-Zeitschr., 1, 60 (1906). 


TECHNICAL APPLICATIONS 213 


making of mirrors, like those of silver, represents an analo- 
gous dispersoid-chemical process.! 


$10. 


The most important applications of colloid and dispersoid 
chemistry are, however, to be found in the metallurgy of 
the alloys — more especially in the metallurgy of iron and 
steel. Because of the newness of the point of view and the 
importance of the subject, I beg you to let me enter into a 
few details. 

As you know, the alloys, like the different steels and 
irons, show differences not only in chemical composition but 
in structure. You are all familiar, for example, with the 
coarse fracture of the ordinary cast iron as compared with 
the microscopic or even sub-microscopic structure of the 
finer grades of steel. You also know that even with con- 
stancy in chemical composition one and the same steel may 
show very different structures and that the nature of these 
structures is much influenced by variations in the tempera- 
tures to which the metal has been exposed, by the suddenness 
with which these temperature changes have been brought 
about, by tempering, by mechanical stresses and by simple 
ageing. It is possible, in other words, to obtain from one 
and the same mixture of iron and carbon a series of disper- 
soids showing different degrees of dispersion which may 
range from the type of the coarsely crystalline to that of the 
microscopically non-crystalline ‘‘solid”’ solution. We dis- 
cover.a series of dispersoids of iron and carbon (and the same 
is true of other alloys) entirely analogous to the series of 
dispersoids which I showed you in the case of sulphur, of 
sodium chlorid, of silicic acid, ete. 

This simple and entirely familiar observation that two 
metals may be mixed into each other with the subdivided 
particles showing different degrees of dispersion, becomes 


1 See V. Konuscutrrer, Koll.-Zeitschr., 12, 285 (1912); further papers 
are listed in the indices of the Kolloid-Zeitschrift from 12 on. 


214 COLLOID CHEMISTRY 


tremendously important in the light of our knowledge of 
the dispersed systems. The technical and physico-chemical 
properties of an alloy are largely dependent upon the size of 
the subdivided particles constituting it. Coarsely structured 
alloys are, as a rule, brittle and non-elastic, while in the 
words of the metallurgist, W. GUrtTuEr, ‘‘a finely granular 
material and the absence of every marked or sharply de- 
fined structure are the signs of a mechanically valuable 
product.’’! 

Even though this relation between degree of dispersion 
and technical properties has been generally recognized, it 
has by no means been regarded as of the importance which 
modern authors, among whom I should like to count myself, 
have assigned to it. Two other physico-chemical principles 
are still assumed to be of chief importance, the phase rule 
and vAN’T Horr’s concept of the solid solution,? while the 


1 See W. GurtLeR, Handbuch der Metallographie, 1, II, 450, Berlin, 1913. 

2 In spite of my great admiration for the progress that has been made in 
metallography through the introduction into this field of the concepts of 
chemical equilibrium, the phase rule and van’t Horr’s notion of solid 
solution, I cannot help emphasizing the need of caution in all this, for these 
concepts are all based upon the truth of certain assumptions. The concept 
of equilibrium, for example, assumes that we deal with states of equilibrium 
which under experimental conditions may be reached from either side. It 
is, however, characteristic of alloys (like the steels) that the changes taking 
place in them never come to a stop. The belief that true equilibria are 
attained in these solid mixtures therefore lacks support. It seems worthy 
of note that we find in the metallic world no reference to the fact that Gress’ 
phase rule may be applied only if the conditions presupposed existent by 
the author of the rule are really present and since these are not satisfied in 
the majority of the alloys, application of the rule is therefore forbidden. 
Gipb’s rule is valid only for equilibria in systems in which, to use the words 
of WiLLARD G1BBs himself, the energies existent in the surfaces of the phases 
composing the system may be ignored. It is valid, in other words, only 
for macro-heterogeneous systems. ‘These conditions are not fulfilled in the 
case of the dispersoid alloys, particularly not in those which are technically 
most important and in which the degree of dispersion is particularly high. 
Gipss’ phase rule cannot be applied to these alloys any more than it can 
be to ordinary liquid colloids. Metallurgy, moreover, confines itself to 
solid solutions in the sense of van’t Horr, in other words, to molec- 
ularly dispersed solutions. It practically ignores, therefore, the ques- 
tion of degree of dispersion. The technically far more important colloid 


TECHNICAL APPLICATIONS 215 


relation between size of granule and physical properties has 
been regarded as of only secondary significance. 

Numerous texts are, of course, available which deal with 
these obvious physical relationships, though their number 
is greatly exceeded by such as deal with the more purely 
chemical aspects of the problem. What has been lacking 
has been proper emphasis upon the tremendous importance 
of size of granules to physical properties and relative lack of 
importance of the specific chemical composition of the alloy. 
There was missing, to put it briefly, a proper correlation of 
the phenomena observable in these fields with the analogous 
phenomena observable in other fields and in which these 
relationships could be followed in simpler and more general 
fashion. This correlation is splendidly made when it is 
recognized that the problems of the physical chemistry of 
the alloys is that of the physical chemistry of the dispersed 
systems and of colloid chemistry. For what is the study of 
the dispersed systems but that of the relation between size of 
granules and physico-chemical properties, and what rs colloid 
chemistry but the science of a special subdivision in this realm? 
And have we not found that changes in degree of dispersion 
constitute a factor which brings about incomparably more 
radical variations in the physico-chemical properties of any 
dispersoid than are observed, for instance, between the 
properties of different steels? Colloid chemistry has shown 
us that the physico-chemical properties of the dispersoid 
change with changes in the size of the dispersed granules 
and that these differences in degree of dispersion are the 
chief factors of moment, distinguishing the colloids in this 
way from the molecular solution on the one hand and the 
coarse suspension on the other, while at the same time 
serving to unite the two. From the teachings of colloid 


solid solutions, to the wide distribution of which I called attention some 
twelve years ago [see, forexample the article of P. P. von Wemmarn, Koll.- 
Zeitschr., 7, 35 (1910)], have to the present time received no conscious treat- 
ment in metallography excepting by C. Brenrpicks (references to whose work 
are given later). 


216 COLLOID CHEMISTRY 


chemistry, it cannot be gainsaid that in metallurgy, too, the 
relation between size of granule and properties belongs to 
the most important and most widespread of all the relation- 
ships with which we are familiar in modern physical chem- 
istry.! 


811. 


If we try to classify according to degree of dispersion the 
different structural elements which enter into the iron- 
carbon compounds, we may begin with the three metarals, 
iron or ferrite (with which may be included its various allo- 
tropic forms) iron carbid (cementite) and carbon. Carbon 
appears in the dispersoid series: graphite, tempering carbon, 
hardening carbon. 

Graphite is a coarsely dispersed carbon; hardening car- 
bon, an extraordinarily highly dispersed one. Accord- 
ing to prevalent hypothesis, the carbon in this material is 
supposed to exist in molecular, solid solution. Tempering 
carbon occupies a middle position in the matter of degree of 
dispersion and physico-chemical properties. The allotropy 
of these carbons is a dispersion allotropy. The degree of 
dispersion in hardening carbon is, perhaps, not entirely 
molecular. It apparently occupies a transition point be- 
tween colloidally and molecularly dispersed solid solutions. 
According to some of my still unpublished experiments, the 
so-called E@crrtz solutions of this hardening carbon, as 
obtained by solution of steel in dilute nitric acid, are obvi- 
ously colloid in nature. They scarcely dialyze, are ultra- 
microscopically heterogeneous, etc. 

Iron carbid or cementite also presents different depres of 
dispersion. Primary cementite is coarsely crystalline, seg- 
regated cementite somewhat finer, perlitic cementite finer 

1 W. GURTLER in his admirable handbook lays more emphasis upon these 
important relations between the degree of dispersion and physico-chemical 
properties than any other author, though, to my mind, still not enough. 
See in this connection the paper of P. P. von Wrrmarn, Internat. Zeitschr. 


f. Metallographie, 65 (1911), as well as the remarks of W. GUrTLEr following 
this paper. 


TECHNICAL APPLICATIONS 217 


still. The finest of this series of materials may show so 
slight a structure that, in the words of W. Gurruer, it 
represents ‘“‘an almost molecularly dispersed solid solution.” 
Even ferrite presents different degrees of dispersion, 
though the relationships here are complicated by the fact 
that iron by itself appears in a number of different forms 
which are allotropic in the ordinary sense of the word. 


$12. 


After these primary elements we need to consider the 
innumerable secondary ones which result from their com- 
bination. Of especial importance are mixtures of ferrite 
and carbon and of ferrite and cementite. From the many 
illustrations available, I touch upon a group which is in- 
teresting because of its importance in the technology of 
steel, and because it concerns a field which has been analyzed 
dispersoid-chemically by the Swedish investigator, C. BENrE- 
picks.! I refer to the series obtained when steel is chilled 
at different rates, namely, austenite, martensite, troostite, 
osmondite, sorbite and perlite. In this series austenite and 
martensite (perhaps, also, hardenite) are the most highly 
dispersed. They are probably molecularly dispersed mix- 
tures obtained when the chilling is brought about very 
rapidly. Perlite is obtained with slower cooling. It shows? 


1 ©, Benepicks, Zeitschr. f. physikal. Chem., 52, 6 (1905); Jour. Iron and 
Steel Inst., 352 (1905); IKsoll.-Zeitschr., 7, 290 (1910). In studying the 
well-known text on siderology of H. von JUprner, without being aware of 
the contributions of Brnrpicks, I arrived, in 1909, at a colloid-chemical 
' view of metallographic processes, the theoretical and experimental results of 
which were published, in part, in some of the papers which I have cited. 
Not until later, after I had made a whole series of colloid-chemical experi- 
ments, as with the Eacrrrz’ solutions, did I run across the short first paper 
of C. Brnepicks, cited above, in which he expressed in very clear fashion 
a part of these colloid-chemical notions. It was after this that I wrote to 
C. Brenepicks asking him to collect his views and to allow me to publish 
them in the Kolloid-Zeitschrift. 

2 Beautiful illustrations of perlite structure may be found in the article 
of C. Benepicks, Koll.-Zeitschr., 7, 290 (1910). The structure reminds one 
strongly of LissnGan@’s rings, 


218 COLLOID CHEMISTRY 


characteristic lamella-like deposits of cementite which are 
easily visible microscopically and sometimes even macro- 
scopically. Perlite is, in other words, a relatively coarse 
dispersoid. Between these extremes of martensite and 
perlite are found those which bridge the gap in perfectly 
smooth fashion, namely, troostite, osmondite and sorbite. 
It is for this reason that BENEDICKS came to the conclusion 
that these intermediates represent solid colloid solutions. 
Troostite is evidently a colloid solution of cementite in ferrite; 
perlite 1s the coarsely dispersed or coagulated product of this 
cementite-ferrite sol. 

Faith in the existence of these colloid intermediates seems 
not only justified but absolutely necessary when it is re- 
membered that molecularly dispersed or coarsely dispersed 
metarals in moving from the one class into the other must pass 
through the colloid realm. 'The only question at stake is 
whether it is possible to fix the material at the moment in 
which it is passing through this middle region. 

But to this end solid dispersion media and the sensitive- 
ness of alloys to external conditions obviously offer the 
best possible opportunities. Troostite is best produced 
through careful reheating of steels. This is no doubt be- 
cause in this fashion a condensation is brought about of the 
previously molecularly dissolved carbon or cementite parti- 
cles contained in the original martensite or austenite. The 
heating process and its results are, in other words, analogous 
to the repeatedly discussed conditions best. designed to yield 
colloid solutions of gold, of sodium or of sulphur from their 
molecularly dispersed solid solutions. As known to every- 
one, carefully regulated increases and decreases in tempera- 
ture are constantly used in the manufacture of metallic 
products to get these to show as fine a grain as possible. 

. But of especial importance are the relations which exist 
between these differently dispersed states! of the metarals 

1 W. GURTLER (1.c.) has also repeatedly emphasized that not a definite 


chemical or physical property, but rather a definite structural state — in 
other words, a definite relationship of the differently dispersed metarals 


TECHNICAL APPLICATIONS 219 


and the technical and physico-chemical properties of the 
resulting metallic products. As emphasized by C. Brenr- 
DIcKs, the efforts of the metallurgist are constantly directed 
toward the obtaining of a maximal elasticity and toughness 
of his technically important alloys. To accomplish this he 
tries, as far as possible, to get all the constituents of his 
alloys into a colloid degree of dispersion. As BENEDICKS 
puts it, “‘a correctly produced hair spring consists of troo- 
stite; the cry is always for steel rails consisting of sorbite; 
the tendons of technology, our steel cables, are, like the 
tendons of the human body, colloid in structure. The 
dispersion medium in all these instances is a distinctly 
crystalline body, the separate crystals of which man has for 
ages past tried to keep just as small as possible.” 

What is interesting in conjunction with these observations 
is that many of the most important technological properties 
of the metallic alloys attain their optimum in a region of 
medium or colloid degree of dispersion and not, for example, 
in a higher or molecular one. The property of hardness, 
for instance, grows steadily with increase in degree of dis- 
persion, yet a suddenly cooled, ‘‘hard as glass,” austenitic 
steel becomes so brittle that the progressive increase in 
hardness soon has a limit put upon it. Elasticity, tough- 
ness, modulus of rupture, rate of solution in dilute acids, 
coloration intensity of structure through iodin or picric 
acid, all these and many other properties attain their opti- 
mum in a region of medium grade of dispersion. Such facts 
regarding the alloys parallel the variations in color, opacity, 
viscosity, etc., which we discussed in our second lecture, 
where we also discovered that a medium or colloid degree 
of dispersion allowed these properties to appear in most 


to each other — is characteristic of the technically important iron alloys, 
and that many of the names given to structural elements do not, as a matter 
of fact, refer to different individual structural elements, but cover their state 
of subdivision and of admixture with each other. Of the more recent papers 
see J. CzocHRALSKI, Stahl und Eisen, 36, 863 (1916); W. Gtrruer, Zeitschr. 
f. Metallkunde, 11, 61 (1919). 


220 COLLOID CHEMISTRY 


intense form. The curves expressive of the solution velocity 
of a series of alloys (martensite, osmondite, troostite, per- 
lite), in which a maximum is observed in the middle, the 
curves which show that in cast iron there exists an optimum 
for the degree of dispersion of the graphite and that this 
corresponds with a maximum of carrying strain,! these are 
curves thoroughly familiar to every colloid chemist. 

But other metallurgical phenomena find a parallel in 
colloid-chemical ones. Certain steels on cooling show a 
viscosity maximum somewhere below 1700° C. With fall- 
ing temperature they do not become progressively more 
viscid as do normal liquids but exhibit a great increase in 
viscosity followed by a sudden decrease.2 This is the same 
behavior which we encountered previously in the separa- 
tion phenomena of hydrated emulsoids, of critical fluid 
mixtures, of crystalline liquids and of molten sulphur. The 
parallelism justifies the conclusion that in the case of the 
molten steels there also occurs at the temperatures under 
discussion a separation in highly dispersed or colloid form 
which with still greater lowering of temperature yields a 
coarsely dispersed system. It is possible that the so-called 
pseudoeuctectoid melts? are like these liquid steels. Accord- 
ing to R. Lorenz it is possible to make colloid solutions in 
molten salts, the so-called ‘‘pyrosols.”’ 

A further analogy of metallurgical behavior to that of the 
colloids is seen in the fact that the melting point of ordinary 
grey iron, for example, is decidedly higher than its solidifica- 
tion point. The same phenomenon may be observed in 
any gelatin gel. The explanation in both cases seems to be 
that during and after solidification, the particles grow to 
larger aggregates; and the melting point of large particles 
is decidedly higher than that of more highly dispersed ones. 

‘It must also be pointed out that when the carbon content 

1 See W. Gtrrier, Handbuch, l.c., 1, II, 308 (experiments of Hayn and 
LEYDE). 

2 See W. GurRTLER, l.c., 131. 


3 See W. GURTLER, l.c., 188. 
4 See W. GURTLER, l.c., 186. 











TECHNICAL APPLICATIONS 221 


of a steel reaches a concentration of about 0.45 percent, a 
peculiar change in structure! takes place. While a so-called 
steel structure is characteristic of the higher carbon contents, 
there appears a so-called granular structure when the con- 
centration of carbon is less. An analogous difference in 
structure characterizes, for instance, the setting of a con- 
centrated and a dilute gelatin if this contains a little alecohol.? 

I beg to conclude these remarks on the relation of colloid 
chemistry to metallurgy by pointing out that the phenomena 
of ageing, of fatigue and of distortion in alloys, whatever 
their kind, all tend in general toward a decrease in degree of 
dispersion — a behavior analogous, therefore, to that dis- 
cussed previously as characteristic of these same phenomena 
in colloid systems. It is as true of a steel as of a colloid that 
the slightest alterations in external or internal conditions 
bring about great changes in its state. Things are no more 
at rest in a steel than in any colloid mixture. 


§13. 


This brings us to the important and varied applications 
which have been made of colloid chemistry to the organic 
industries and the technical arts. We are Justified, as a 
matter of fact, in designating these as the colloid industries 
or the colloid-chemical arts. 

To begin with the latter heading, we may take up, as of 


first importance, the processes of dyeing and tanning.* Of 


the many available illustrations I must again choose an 


1 See W. GURTLER, l.c., 384. 
2 See, for example, Wotraana Ostwap, Grundriss der. Kolloidchemie, 


| 1. Aufl., 350, Dresden, 1909. 


8 For a discussion of dyeing from a colloid-chemical point of view, see 
J. Pevet-Jourvet, Die Theorie des Farbeprozesses, Dresden, 1912, as well 
as his numerous papers in the Kolloid-Zeitschrift. Among newer studies in 
this field see those of R. Hauer in the Kolloid-Zeitschrift and the Kol- 
loidchemische Beihefte. For the colloid chemistry of tanning, see the re- 
views of E. Stiasny, Koll.-Zeitschr., 2, 257 (1908) and Cur. NEuNER, ibid., 
8, 329 (1910); 9, 65, 144 (1911). Numerous other papers upon these sub- 
jects by other authors especially W. MOLLER may be found in the Kolloid- 
Zeitschrift and in the Kolloidchemische Beihefte. 


oo, COLLOID CHEMISTRY 


arbitrary few. In the first place, we must not in these 
fields make the mistake of considering everything we see as - 
exclusively colloid-chemical in nature. Dyeing and tanning 
are complex procedures which aim at definite end results 
but they make no assumptions regarding the nature of the 
processes which are to lead to such results. A dyeing or 
tanning effect may be accomplished by very different 
methods, in the list of which appear many non-colloid- 
chemical ones. Some colloid-chemical processes must, of 
course, always appear as long as the material to be dyed or 
to be tanned is itself colloid in nature. In practice this is 
nearly always the case, for not only are textiles and hides 
typical gels or mixtures of such, but just as every chemical 
reaction into which a colloid component enters must in 
consequence show certain colloid-chemical peculiarities, just 
so must even the so-called “‘purely’’ chemical dyeing and 
tanning processes — as dyeing with an oxidizing agent or 
tanning with formaldehyde — show colloid-chemical pecu- 
liarities. But in many cases there is added to this colloid- 
chemical component resident in the substance itself that 
second one due to the fact that the dye bath or the tanning 
solution is colloid in nature. An unexpectedly large num- 
ber of organic dyes as employed under factory conditions are 
colloidally dissolved and the same is true of nearly all the 
vegetable tanning materials, such as tannin and the dif- 
ferent bark extracts. Many mineral substances as used 
in tanning are also colloid, as chromium hydroxid, sulphur, 
etc. In all these instances we therefore deal with reactions 
between at least two colloids, and to these is often added the 
effect of several more colloids. Thus in certain dyeing 
processes organic colloids like tannin or inorganic colloids 
like aluminium hydroxid are often added. In all such col- 
loid reactions purely chemical ones come to play a decidedly 
minor réle when compared with the adsorption effects and 
all their possible secondary reactions which we discussed in 
a previous lecture. As in the simpler examples of adsorp- 
tion, we need in the adsorption of dyes and of tanning 


TECHNICAL APPLICATIONS 28 


materials to recognize a whole series of different reactions as 
taking place side by side. In these there may predominate 
at one time electrical effects due to electrical differences 
between substrate and materials to be adsorbed, at another 
time surface tension effects, at a third, chemical ones. The 
literature abounds, therefore, in electrical, mechanical, 
chemical and other theories of dyeing and of tanning. 
These all tend to err in that they incline to place some one 
of these theories over and against some other; but as we 
found in our discussion of adsorption, the changes char- 
acteristic of this may be brought about in several totally 
different fashions, in which any one may be quite as impor- 
tant as any other. 

Of great importance in dyeing is not only the accumula- 
tion of the dye in the material to be dyed, but its fixation 
in the material. The dye must be united in irreversible 
fashion to the fiber. The mistake is often made of alleging 
that adsorption alone cannot explain ‘‘fast’’ dyeing since, 
by definition, adsorption is always a reversible process. 
This view forgets that it all depends upon the intensity of 
the adsorption whether on rinsing in the pure dispersion 
medium some of the adsorbed material will again go back 
into solution or not. If the adsorption is so intense that 
practically all of the dye is taken out of the dye bath — and 
this is obviously the most economical method of dyeing — 
then none of the adsorbed dye is likely to pass out of the 
dyed material into the pure water, for this represents a still 
more dilute solution of the dye than did the almost de- 
colorized dye bath. which was in equilibrium with the con- 
centration of the dye in the dyed textile. While adsorption 
. alone may therefore lead to ‘‘fast’’ dyeing, secondary 
changes like polymerizations, secondary decompositions, 
direct chemical unions between fiber and dye, etc., often 
come about or are brought about to yield the ultimate 
‘‘fast”’ result.} 


1 See the main text for a discussion of secondary reactions consequent 
upon adsorption. 


224 COLLOID CHEMISTRY 


In similar fashion the process of tanning — the process, 
in other words, of changing hide into leather — can not be 
explained through mere adsorption of the tanning materials. 
The taking up of the tanning materials is only one part of 
the process though one which regulates as a rule all the 
further changes in state of the colloid substrate. If a col- 
loid chemist is asked to say what constitutes the chief dif- 
ference between hide and leather, he points out the follow- 
ing. Hide and leather are both structured gels; they are, 
in other words, collections of fibrils composed of collagen and 
other proteins arranged in that peculiar fashion determined 
originally by the anatomical structure of skin. In the 
untanned hide the fibrils are gummed together and contain 
much water. In leather they are more isolated and, at 
the same time, more strongly dehydrated or coagulated. 
If the more important stages of tanning are considered from 
this colloid-chemical point of view, it is clear that the initial 
swelling induced in the hide through immersion in acid 
(preferably an organic acid) amounts to this, that the in- 
dividual fibers are ‘“‘unravelled”’ as much as possible. The 
original gumming of the fibers, aggravated by any drying 
to which the hide may have been subjected is overcome. 
As a matter of fact, the cement substance holding the fibrils 
together may actually be hydrolyzed and thus brought into 
solution. This fluffing of the hide substance opens a path 
for the better entrance of the tanning materials and exposes 
a larger surface for adsorption. Complete tanning of a 
hide presupposes that the surface of every fibril will in this 
fashion be brought in contact with the tanning materials. 
But union with the tanning materials is also favored through 
such preliminary treatment just as mercerization (in other 
words, the fluffing of the cotton fibrils through previous 
swelling in alkalies) helps cotton to take up dyes. A sec- 
ond group of changes in state now takes place in hide. 
The fibrils are coagulated, that is to say, reduced to a coarser 
state of subdivision accompanied by a giving off of water. 
The trick of the tanner is the avoidance in this coagulation 


TECHNICAL APPLICATIONS ae 


of a fresh felting of the isolated fibrils — this occurs fre- 
quently, for instance, in iron tanning. On the other hand 
the coagulation must not be carried so far that the fibrils 
lose all connection with each other and the leather be over- 
tanned, in other words, be made brittle. 

This colloid-chemical definition of tanning as a limited 
swelling of the hide fibrils followed by their coagulation holds 
also for the ‘‘physical”’ processes of tanning seen in chamois 
tanning. In this the fibers are also first isolated either 
through the mechanical manipulation incident to the in- 
troduction of oils and fats into the skin; or through the 
colloid-chemical effects produced by the oxidation products 
constantly present in the fats and oils used. Through the 
latter the fibrils are then subsequently also coagulated and 
dehydrated. 


$14. 


Industries which may in the true sense of the word be 
called colloid industries are seen in the group of those which 
work with cellulose. Pure cellulose is already a typical gel 
possessed of a beautiful ultramicroscopic structure; it shows 
typical swelling phenomena and on solution yields the highly 
viscid liquids which are characteristic of the hydrated — 
emulsoids. It may be precipitated from these solutions 
through neutral salts or through dehydrating agents like 
alcohol. 

These phenomena are all reversible. Of especial interest 
is a series which is irreversible in nature. I refer to the 
processes of parchment manufacture and of mercerization. 
Cellulose (in the form of filter paper or cotton, for example) 
swells in acids and alkalies of medium concentration more 
than in water, just as does gelatin or fibrin under similar 
circumstances. But following this treatment there occur 
various secondary changes which upon removal of the 
alkali and drying of the gel leave this in a much more highly 
dispersed and voluminous state than before. We have 
before us, then, parchment paper or mercerized cotton, 


226 COLLOID CHEMISTRY 


either of which now shows new optical and mechanical 
properties to correspond with the colloid-chemical changes 
in state that it has suffered. 

Various derivatives of cellulose and various combinations 
of cellulose with other materials in the form of solid solutions 
show typical colloid-chemical behavior. Best known are 
the cellulose gels used for the production of artificial silks 
and various plastic masses. Alkali-cellulose, like alkali 
gelatin, when treated with carbon bisulphid yields a remark- 
able substance known as viscose. From viscose is today 
prepared most of the artificial silk —an industry which 
already has an annual value of $200,000,000. Viscose 
shows all the characteristics of a hydrated emulsoid. The 
whole process of viscose silk manufacture is naturally honey- 
combed with colloid-chemical phenomena. The freshly 
prepared viscose is first aged or ripened before it yields a 
product optimal for spinning. The velocity of this internal 
change in state can be influenced through the addition of 
various substances. The originally fluid fibers must be 
coagulated; to accomplish this end the patent literature is 
filled with such a long series of different processes that it is 
safe to say that this coagulation phenomenon in viscose 
probably represents the best studied of the whole line of 
coagulations in emulsion colloids. These internal changes 
in state must, moreover, be inhibited at a certain time else 
the fiber becomes brittle. The tendency of the fiber to 
swell in water or steam and the consequent weakening of 
the fiber must be reduced to a minimum. The whole in- 
dustry represents an unbroken succession of colloid-chemical 
processes. 

Other cellulose compounds and solutions of them, like that 
of cellulose in ammoniated copper oxid, may be employed in 
‘similar fashion. Artificial silk can also be prepared from 
gelatin, though when this is used other methods of coagula- 
tion must be employed than in the case of viscose. Cellu- 
lose esters may be used to prepare transparent varnishes 
such as are employed upon aeroplanes, and these same 


TECHNICAL APPLICATIONS 227 


materials are used for the production of plastic masses of 
various types... The best known of these plastic masses is 
celluloid, that peculiar and scientifically little studied solid 
solution of cellulose and camphor, the uses of which are 
familiar to every one. Because of its high inflammapility 
numerous cellulose derivatives such as cellon and acetyl 
cellulose ester have been introduced to take its place. 

I should like to call your attention to some further plastic 
masses which are also colloid gels. Galalite is nothing but 
a casein, so treated that it no longer shows any powers of 
swelling. Bakelite is a condensation product derived from 
phenol in the presence of alkali and formaldehyde. It may 
be obtained in all states of aggregation varying from a soft, 
jelly-like material to a brittle, almost stony, resin-like 
mass. Bakelite is interesting colloid-chemically because 
it is a typical isocolloid, that is to say, a dispersoid in which 
the dispersed phase and the dispersion medium are polymers 
of each other. We shall shortly come upon another illus- 
tration of this group of materials. 


$15. 


The manufacture of rubber? represents another typical 
colloid industry. A number of colloid and dispersoid- 
chemical processes are observable even in the first prepara- 
tion of crude rubber. Soft rubber comes from latex, which 
consists of a dispersion of tiny droplets of tenacious liquid 
in an albuminous serum. Just as in ordinary milk, it 1s 
probable that the protein surrounds the soft rubber globules 
as an adsorption membrane; in other words, as a so-called 
‘“haptogenic membrane.” This protein seems to play a 

1 Regarding the manufacture of plastic masses and of artificial textiles — 
especially such as are derived from cellulose — see the review of J. G. BELTzER, 
Koll.-Zeitschr., 8, 177, 313 (1911). 

2 Nearly all the newer papers covering the manufacture of rubber from 
a colloid-chemical point of view may be found in the original in the Kolloid- 
Zeitschrift and the Kolloidchemische Beihefte under the names of WOLFGANG 
Ostwatp, F. W. Hinricusen, B. Bysow, D. Spence, D. Dirmar, P. Scui- 
DROWITZ, etc. 


228 COLLOID CHEMISTRY 


great role not only in the original coagulation of the latex, 
but also in giving to soft rubber its characteristic mechanical 
properties. The protein is apparently of the group of the 
globulins, for latex is commonly coagulated through the 
addition of carbonic acid or of distilled water, both of them 
typical coagulation procedures for the group of the globu- 
« lins. In fact, all the various coagulation methods employed 
seem to be nothing but protein precipitation methods. 
Their proper employment seems to be a fundamentally 
important feature in the whole process of soft rubber prep- 
aration.! 

Freshly prepared raw rubber shows a marked syneresis. 
It squeezes off a protein-rich serum. A proper separation 
of this material is of great importance, since it reduces the 
possibilities for bacterial growth. 

Raw rubber swells tremendously in different organic 
solvents, as I have already shown you. Upon the appli- 
cation of heat, and through mechanical agitation, a large 
part of the soft rubber may be made to go into “solution.” 
In this process the remnants of coagulated protein left in 
the soft rubber undoubtedly play an important part, since 
they serve to counteract the tendency of the swollen rubber 
particles to go into solution. These solutions of soft rub- 
ber behave like typical solvated emulsoids. They show 
great absolute viscosities, great relative increases with rise 
in the concentration of the colloid, phenomena of ageing, 
etc. All the observed phenomena are complicated through 
the presence of traces of the protein. Through the pres- 
ence of such may apparently be explained the fact? that 
upon addition of acids there occurs a rapid decrease in the 
viscosity of soft rubber sols. This viscosity of the soft 
_rubber is of great importance in determining the life of 
the solid rubber. It has been found that, as a rule, a rela- 
tively high viscosity yields a lively rubber; in fact, under 
certain circumstances this parallelism is a quantitative 


1 See also in this connection Koll.-Zeitschr., 13, 324 (1913). 
2 D. Spence, Koll.-Zeitschr., 14, Heft 6 (1914). 


TECHNICAL APPLICATIONS 229 


one in that the viscosity value parallels directly certain 
mechanical properties of the vulcanized product like its 
resistance to tear.! 

Of great importance is the process of vulcanization — 
that series of important physico-chemical changes which 
occurs when soft rubber is heated with sulphur or sulphur 
compounds. At least three different types of changes are 
to be distinguished from each other in the process of vul- 
canization: first, the taking up of the sulphur or sulphur 
compounds; second, their fixation; third, the changes in 
state in the rubber following therefrom. 

The nature of the first of these processes is still a matter 
of lively debate. Some maintain it to be a typical ad- 
sorption phenomenon, while this view is cast aside entirely 
by others. How greatly opinions differ is well illustrated 
in two papers which appeared almost simultaneously. In 
one of them the author took it almost for granted that a 
colloid like rubber should show adsorption phenomena; 
in the other, the author concluded that not a single fact 
argued for the importance of adsorption in the process of 
vulcanization. Since I was the first to defend the adsorption 
notion — though I did not by any means hold it to be an 
entirely ‘‘self-evident’’ one—I naturally incline to the 
view that the taking up of the sulphur is really an adsorption 
process. This view is supported not only by the fact that 
the process is governed by the adsorption law, but by the 
results of D. SpENcE’s extraction experiments which show 
that the amount of sulphur adsorbed is inversely proportional 
to the reciprocal function of the free sulphur.? After the 


1 See A. von RossEM, Kolloidchem. Beihefte, 10, 117 (1918). 

2 In spite of these corroborative findings by D. Spence himself, he still 
maintains in his recent papers that adsorption processes play no important 
role in the process of vulcanization. Some misunderstandings have served 
to complicate the discussion. A criticism of SpmENcr’s most recently ex- 
pressed views is left for the future. C. Harriss [Koll.-Zeitschr., 19, 1 (1916); 
Untersuchungen iiber die natiirlichen und kiinstlichen Kautschukarten, 
Berlin, 1919] has recently come to the conclusion that the taking up of sul- 
phur by rubber is largely an adsorption phenomenon. A review of the 
various theories of vulcanization may be found in G. van ITERSEN, Kol- 
loidchem. Beihefte, 12, 252 (1920). 


230 COLLOID CHEMISTRY 


sulphur has been taken up by adsorption an important 
chemical union may take place. But there also occurs at 
this time an important change in the state of the rubber 
to which we shall return later. 

Permit me to add a few words regarding ‘‘synthetic”’ 
rubber, this newest of the children of organic chemical. 
industry. This material, as you know, is made by polym- 
erizing isopren and related hydrocarbons. In the process 
of polymerization the isopren passes through the several 
states of a slightly viscid liquid to a stiffer one, to end in 
a hard, crumbly mass. It is between the two extremes 
that we obtain a product most like natural rubber. This 
product may be separated as a gelatinous mass from the 
monomeric mother substance through the addition of 
alcohol. The synthetic rubber obtained by these methods 
may be used as a class-room example of an isocolloid or 
isodispersoid. In the first stages of polymerization the 
dispersion medium consists of the monomeric mother sub- 
stance in which float the polymerized particles; in the 
medium and higher concentrations there seems to occur 
a transformation which results in the monomeric compo- 
nent becoming the internal phase while the polymeric 
form surrounds it as a gelatinous structure. 

The synthetic product does not, for the most part, possess 
the life or vulcanization possibilities of natural rubber. 
The attempt has therefore been made to overcome this 
defect by introducing protein and similar colloids into the 
artificial substance though not as yet with entirely satis- 
factory results.!_ Interest in synthetic rubber has recently 

1 The lack of success following present day attempts to give to synthetic 
rubber the excellent qualities of the natural product or even better ones 
through the introduction of protein into the rubber would seem to depend 
’ upon the difficulties incident to such introduction in sufficiently dispersed 
form. This end might be attained more easily if the synthetic product were 
first made into an aqueous emulsion, in other words, were first made into an 
artificial latex — a procedure mimicking, ‘for example, the newer methods 
used in the manufacture of the margarines in which an artificial milk is also 


first made and from which there is then produced the butter-like product. 
I know, however, from personal experiments and those of adepts in the field 


TECHNICAL APPLICATIONS 231 


subsided somewhat because of the great fall in price of the 
natural product. This defection is, I think, not merited. 
It seems to me that interest in artificial rubber does not today 
center in the possibility of finding a substitute for the natural 
product, but more in the possibility of making a decidedly 
better one. The situation is not unlike that obtaining in 
the case of the metallic alloys. We did not in this field 
rest content with the utilization merely of such metallic 
mixtures as nature furnished us. Only as we have treated 
these in various fashions or have added other materials to 
them have we obtained the ‘‘noble”’ alloys which now serve 
us. It seems to me as though what we lack in the manu- 
facture of a satisfactory synthetic rubber is the presence in 
it of a second heterogeneous substance like protein, just as 
we need carbon present in iron to give us steel; in fact this 
analogy between rubber and steel seems to me to be more 
than a superficial one. From a synthetic ‘‘noble rubber” 
materials might be produced showing properties far superior 
to our present day ones produced from the ‘“‘natural”’ 
product. 

What is the nature of the colloid-chemical changes which 
mark the change from raw rubber to the vulcanized? Can 
they be regarded collectively from some such general point 
of view as can tanning? Whatever is said must be applic- 
able, of course, to natural and to artificial rubber for both 
ean be vulcanized even though the resultant products are 
not equally good. To the colloid chemist, synthetic and 
natural rubbers are zsocolloids, in other words, gels in which 
the structural elements consist of polymerized molecules 
in association with simpler molecules of the same chemical 
composition, — polymeric particles, for example, dispersed 
in a phase of monomeric ones. Natural rubber is possessed 
of a second type of heterogeneity because of the dispersion 
within it of protein, resin, etc., in finely divided form. 

The surface capable of adsorbing sulphur is represented 


that it is not an easy matter to make highly dispersed, stabile emulsions of 
rubber in an aqueous dispersion medium. 


232 COLLOID CHEMISTRY 


by the surfaces of the original latex droplets. Just as in 
the case of the fibrils in hide, these droplets are originally 
gummed together and must first be “‘loosened up”! either 
through heat (in the process of hot vulcanization) or through 
some substance which brings about their swelling (in the 
process of cold vulcanization). In either case the actual 
vulcanization, that is to say, the process of taking up sulphur 
seems to displace the equilibrium existent between the 
elements in the rubber, with their different degrees of 
polymerization, in the direction of an increase in the more 
highly polymerized fractions. Were a mass of rubber thought 
of as a series of fused latex droplets but with the outside of 
the mass more highly polymerized than the more liquid 
inside, vulcanization would represent a progressive solidi- 
fication of the sphere. Whether the polymerization is 
thought of chemically or colloid-chemically (as for example 
a coalescence of particles similar to the ‘‘condensation ”’ 
observed when a rubber sol is precipitated with sulphur 
monochlorid) is a question of secondary importance. The 
process of vulcanization, as the process of tanning, can also 
be carried too far,—and the rubber be made brittle. 
Vulcanization is therefore not unlike tanning, only in the 
case of rubber it is an isocolloid which is coagulated. In- 
stead of being dehydrated the particles of rubber suffer a 
colloid-chemical condensation, in other words, the number 
of coarsely dispersed particles with their higher viscosities 
is increased. But, just as in tanning, a definite structure 
possessed by the gel as a whole must not be destroyed. 
Loss of such, as induced through too violent milling, may 
take all the ‘‘life” out of rubber.? 4 


1 The lack of well developed internal ‘‘adsorption surfaces’? may be to 
- blame for the difficulties incident to the vulcanization of synthetic rubber, 
another fact which emphasizes the advisability of trying again to bring 
synthetic rubber into the latex state, or of vulcanizing it after it has gone 
through such. 
2 Space does not permit me to discuss the bearing of this colloid-chemical 
concept of vulcanization upon the related views of S. AxELRop, G. Brrn- 
STEIN, F. Krrcnyor, C. Harriss and others. I can only point out that 


TECHNICAL APPLICATIONS 233 


A word is necessary regarding vulcanization by exposure 
to radiant energy like ultra-violet light... This procedure 
also shows many analogies to the physical phenomena seen, 
for example, in the coagulation of proteins and to the 
chemical ones observed in the photo-polymerization of 
styrol to metastyrol.2 Mention needs also to be made of 
vulcanization ‘‘ without sulphur,’ as when PbO or trinitro- 
benzene (OSTROMYSLENSKY) are used. Raw rubber can 
in these ways also be changed to a vulcanized product though 
its technical value is still low.* It would indeed be strange 
colloid-chemically if sulphur alone could coagulate rubber in 
the manner sketched above. It is more probable that 
the future will bring us as various a series of methods of 
vulcanization as we have today of tanning. _ 

A word, too, regarding the so-called vulcanization ac- 
celerators, — materials which in the past few years have 
been much discussed and assumed considerable technologi- 
cal significance! These materials are, for the most part, 
neutral or basic nitrogenous compounds like di-methyl- 
anilin or hexamethylentetramin. ‘The addition of a few 
percent of these substances not only shortens the time 
the view is not universally tenable that vulcanization represents a ‘‘depolym- 
erization”’ followed by a “polymerization.”’ It is hard to imagine how such 
antagonistic effects can be produced when the external conditions remain the 
same, depending only upon the lengths of time that the rubber is acted upon. 
The concept sketched above is more specific in that heat and the swelling 
medium are supposed to produce a depolymerizing action only upon the 
cement substance existent between the original latex droplets or, expressed 
more accurately, only upon the surfaces of these structural elements. Once 
the surfaces are thus exposed, the sulphur can be adsorbed producing an 
opposite, namely, a polymerizing effect. In harmony with this concept is the 
observation of D. Spencr, among others, that milled and unmilled rubber 
bind the same amount of sulphur in spite of the great mechanical differences 
observable in the products. Extremes excepted, milling can not expose more 
adsorption surface than can heating but the mechanical properties of the 


vulcanized product can be influenced just as the mechanical properties of a 
gelatin gel are changed when this is worked through a sieve and then pressed 
together again. 

1 See G. Bernstetn, Koll.-Zeitschr., 11, 185 (1912); 12, 193, 273 (1913). 

2 See the dissertation of G. PosnsaK, Das Metastyrol, Leipzig, 1910. 

3 See, for example, B. E. BunscHoTen, Koll.-Zeitschr.. 23, 25 (1918). 


234 COLLOID CHEMISTRY 


necessary for vulcanization by three-fourths but improves 
the mechanical properties of the product and its keeping 
qualities. It is well known that our German synthetic 
rubber could be made into useful products during the war 
only through the addition of these substances. Not so 
well known is the fact that my brother, WautHER Ost- 
WALD, and I were the first (in 1908) to recommend their 
use and to patent the process.1. How these substances act 
colloid-chemically (aside from their chemical effects of re- 
tarding oxidation) whether, for example, they have a de- 
polymerizing action like heat and materials which bring 
about swelling or whether they act as coagulants, is still 
an open question. 


1 German patents 221310 (Nov. 1, 1908) and 243346 (Dee. 1, 1909). They 
cover particularly the addition of nitrogenous compounds to increase its 
lasting qualities. G. D. Kratz [Jour. Ind. Eng. Chem., 12, 318 (1920)] 
expresses surprise that ‘the two Ostwalds failed to note’ the accelerating 
action exerted by these substances. Since the story of these patents may 
serve as a warning to others who make “‘scientific’’ discoveries, it may be well 
to tell it. The idea of stabilizing rubber and protecting it against depolymer- 
ization, oxidation, etc., was the outgrowth of biochemical experiments on 
the suppression of autoxidation. Laboratory experiments showed rubber to 
behave as expected. But as we did not have at our disposal the facilities 
for working out our experiments on a technical scale, the best known of the 
Berlin “rubber’’ laboratories was asked to test out the matter. This group 
of “experts” on the basis of their experimental studies concluded that the 
procedure had no practical usefulness. Following this verdict we foolishly 
dropped the patents already granted us. But we have had the satisfaction 
of seeing proof in the extraordinary development of this subject since, that 
we thought and observed correctly fourteen years ago. 

D. Spence [La Caoutschouc, 17, 10427 (1920)] in discussing the history 
of this subject states that he used organic substances, such .as piperidin, 
in 1910, in other words, two years before the idea was patented by the Far- 
benfabriken, vormals FrrepricH Barrer. This may be, but that date is 
still two years after that of our patents. The statement that the Diamond 
Rubber Company produced large amounts of piperidin in 1909 may also be 
true, but even this is a year after our patents were granted. It would, more- 
over, have to be proved that the piperidin was not used in such “large 
amounts”? merely for the recovery of rubber from vulcanized materials as 
per the American patent 805903 of 1905 (regeneration of rubber with 
pyridin bases). 


TECHNICAL APPLICATIONS 235 


$16. 


The manufacture of soap' is another colloid. industry. 
Through their viscosity, their powers of gelation and of 
swelling, the salts of the fatty acids again show themselves 
to be typical emulsoids. Of particular interest to the 
colloid chemist are the technical processes involved in the 
salting out of the soaps. Upon the addition of sodium 
chlorid, for example, to the boiled soap solution there 
follows a separation into two liquid layers, one containing 
much soap and little water, the other (besides glycerin) 
much water and little soap. This coagulation phenomenon 
as carried out in practice is one of the most typical and 
probably the most gigantic example that nature offers of 
the precipitation of a hydrated emulsoid. The phenom- 
enon proves on a huge scale that in all emulsoids two li- 
quid phases are really divided into each other. 

The colloid chemistry of the soaps has recently been 
studied in systematic fashion by Martin. H. Fiscuer. He 
has written a whole book from a colloid-chemical point of 
view on these technically important systems showing not 
only how the technical processes of soap manufacture 
may thus be furthered but, conversely, how the colloid 
chemistry of the proteins and the concepts of colloid chem- 
istry itself may be clarified and developed through such 
study.? | 

I can only mention in passing that in the process of 
washing, colloid-chemical processes of all kinds, more par- 
ticularly adsorption phenomena, play a great réle.* 

The manufacture of starch, of glue, of mucilage and of 

1 See especially the reviews of F. Gotpscumipt, Koll.-Zeitschr., 2, 193, 
227, 287 (1908); 5, 81 (1909); 8, 39 (1910); J. LermporrerR, Kolloidchem. 
Beih., 2, 343 (1911). Numerous other papers and references to such appear 
in the Kolloid-Zeitschrift. 

2 Martin H. Fiscuer, Soaps and Proteins, New York, 1921. A German 
translation by J. Matuta is appearing in the Kolloidchemische Beihefte 
and in book form from the press of Theodor Steinkopff, Dresden. 


3 See especially W. Sprine, Koll.-Zeitschr., 4, 161 (1910); 6, 11, 101, 164 
(1911). 


236 COLLOID CHEMISTRY 


sizes of various kinds is also punctuated with colloid-chem- 
ical problems. The whole process of starch manufacture 
consists of nothing but the separation of particles of one 
size, the starch granules, from more coarsely dispersed ones, 
on the one hand, and from more finely divided nitrogen- 
containing colloids, molecularly dispersed salts, etc., on the 
other. 


$17. 


The colloid chemistry of the foods covers a field which we 
tread many times daily. I ask your pardon if I go into some 
detail.! 

Everyone thinks that he knows something about food 
and so far as the phenomena of the subject are con- 
cerned, he does. And yet K. von Voir and Justus von 
Lizepic bemoaned the fact in their day that there existed 
no real science of food. ‘THEODOR Paut has recently made 
the same lament.? Scientific study of the field still contents 
itself with chapters on analysis and the recognition of adul- 
terants but chapters dealing with the preparation of food 
are hardly started. Much as everyone would like to ob- 
tain better food for less money, study of such questions is 
regarded as menial and best left to the cook. A scientific 
study of the preparation of food is considered as only amus- 
ing even in scientific circles. Many laboratories exist for 
the study of the products of human metabolism but there is 
only one institute in Germany for the study of food prep- 
aration and that was established during the war. 

It seems to me that the chapters which are missing from 
our present volumes on food chemistry are those 7n which 
colloid chemistry plays a chief réle — a view which is not mine 
. alone. The following words may serve to illustrate what 
I mean. Because of the wealth of subjects available for 
discussion a choice has to be made and I shall, therefore, 


1 T am using a lecture printed in abbreviated form in Chemiker Zeitung, 


No. 143 (1919). 
2 Tu. Pau, Biochem. Zeitschr., 93, 365 (1919). 


-TECHNICAL APPLICATIONS Zor 


touch only upon the colloid chemistry of three important 
foods, — milk, bread and meat. 

The chemist describes milk as a mixture of fat, proteins, 
sugar, salts and dissolved gases in much water. How would 
the colloid chemist do it? He would ask, first, regarding 
the state of subdivision of the different substances. For 
him milk is a polydispersoid, that is to say, a mixture of 
substances possessed of different degrees of dispersion in 
one and the same dispersion medium, water. As G. Winc- 
NER has pointed out, milk contains all of the three grades 
characteristic of the dispersoids. ‘The fat droplets, for the 
most part of microscopic size, are coarsely dispersed; the 
proteins such as casein, lactalbumin, etc., are colloidally 
subdivided; the milk sugar and salts are maximally dis- 
persed, in other words molecularly or ionically. The 
coarsely dispersed fat is present in non-solvated form; the 
colloid proteins in a swelled or hydrated state. The colloid 
chemist, obviously, is interested in properties quite different 
from those which concern the analytical chemist. Does 
the colloid chemist’s point of view represent anything more 
than a mere listing of some adventitious, unimportant prop- 
erties of milk? As.a matter of fact he can draw from his 
characterization of milk as a polydispersoid some not unim- 
portant conclusions. Materials divided to a point approx- 
imating the colloid state change their properties with ease 
and so vary much when exposed to slight changes in their 
surroundings. The colloid chemist will therefore divide 
milk, from a theoretical point of view, into two fractions; 
into one which is relatively unstabile and which includes the 
colloids and the more coarsely dispersed parts of the sys- 
tem and into a second, which is relatively stabile and in- 
- cludes the molecularly and ionically dispersed portions of 
milk. Experience justifies this classification. While the 
content of electrolytes in milk as measured by freezing point 
determinations, for example, is fairly constant, the fat and 
the protein fractions vary greatly even in the milk of a 
single animal. We shall take up the qualitative variations 


238 COLLOID CHEMISTRY 


in the fat and proteins immediately. From a quantitative 
point of view the importance of distinguishing between the 
two fractions is emphasized in the important law of Cor- 
NALBA and WIEGNER,! according to which the content 
of any constituent of milk is the more constant the higher 
its degree of dispersion. This fact is of importance when 
the question of the adulteration of milk through the addi- 
tion of water is up. Trustworthy determination of such 
adulteration is by no means easy for the percentage of fat 
can not be relied upon because of its normal variations. 
For this reason the calcium chlorid test has steadily grown 
in favor. Upon what does it depend? It divides milk in 
practice into the same two fractions into which we have 
just divided it theoretically. The variable fractions (fat 
and protein) are separated and thrown away, the stabile 
fraction, the serum, being used for the test. Only when 
this stabile fraction shows definite variations from the nor- 
mal may it be said with certainty that water has been added 
to the milk. 

Let us now look at the unstabile fraction. It must first 
be recalled that the fat droplets in the milk of different ani- 
mals differ greatly in size being finest in human milk. The 
importance of this fact for its digestibility is recognized 
both ‘practically and theoretically. According to WeEN- 
ZEL’s law, so important in colloid chemistry, the rate of a 
chemical reaction depends upon the extent of the reacting 
surfaces. The fat droplets of cow’s milk may be made 
smaller by artificial means. Milk thus ‘“homogenized’’ is 
markedly altered in its properties. Not only does it no 
longer cream spontaneously but the butter fat can no longer 
be removed from it by centrifuging; its internal friction is 
increased, etc., etc. 

Similar variation in degree of dispersion and degree of 
hydration (swelling) may be observed in the purely colloid 


1 See, for example, G. WirGner, Zeitschr. f. Untersuch. d. Nahr.- und 
Genussmittel, 27, 425 (1914) where may be found references to his older stud- 
ies. 


TECHNICAL APPLICATIONS 239 


portions of milk, particularly the casein. Human milk 
again shows the most finely divided casein particles; sheep’s 
milk particularly coarse ones. When milk is heated the 
particles are increased in size. Acids, bases and even so- 
dium chlorid bring about similar increases with changes in 
the degree of swellng as may be determined by micro- 
scopic or viscosimetric means. 

Adsorption phenomena also appear in milk. We have 
defined adsorption as the concentration of materials in a 
surface. It is to such concentration that the films formed 
in milk owe their origin. That which forms when milk is 
heated is familiar to you. To such may be attributed the 
easy “‘boiling over” of milk. But adsorption phenomena 
may appear in the surfaces existent within the milk. Such 
surfaces are found, for example, wherever fat droplets touch 
the remaining portions of the milk. The formation of 
‘‘adsorption films’? about the fat droplets is therefore ex- 
pected by the colloid chemist for he knows that protein 
solutions may be rendered protein-free by being shaken with 
mineral oils. The old dairy question regarding the phys- 
ical state of these adsorption envelopes is answered by 
saying that they are gels and as such show the properties 
of both solids and liquids. Their presence has much to 
do with the making of butter and they are responsible, 
too, for the fact that the fat can be extracted only slowly 
and incompletely from milk when shaken with ether. 

The uniformity of mixture in milk may be further dis- 
turbed by sedimentation or creaming. Specifically heavier 
particles sink while lighter ones, like butter fat, rise to the 
top. The velocity of such creaming is also dependent upon 
the colloid-chemical factors of degree of dispersion, degree 
of hydration, viscosity of the medium, etc. 

Let us next consider the coagulation of milk. By coagu- 
lation of a dispersed material we mean reduction in the de- 
gree of its subdivision, a change often accompanied by de- 
struction of the uniform structure of the system. Both 
the coarsely dispersed fat phase and the colloidally dispersed 


240 COLLOID CHEMISTRY 


protein phase of milk may be coagulated. When the fat 
droplets run together the milk has buttered; when the 
colloid casein particles unite, the milk has curdled. In 
order that coagulation of the fat droplets may occur, the : 
gel structure of their adsorption envelopes must be modified 
or destroyed. ‘This is induced, as you know, by allowing 
a definite, medium degree of acid production to come to 
pass in milk or cream. The degree of swelling or the degree 
of dispersion of the proteins constituting the adsorption 
envelopes must be changed in order that the fat droplets 
will run together. W. KrirRcHNER emphasizes the first 
in his textbook on dairying; W. FLEISCHMANN the second, 
in his. To the colloid chemist both factors are prob- 
ably of importance for the two tend constantly to appear 
together in colloids of the type here under discussion. 
The fact that the rate at which butter forms and that the 
quality of the butter are determined by the concentration 
and size of the fat droplets, by the acidity and the temper- 
ature, — all these things have their analogies in the fields 
of pure colloid chemistry. 

The physical state of the clot when milk is coagulated has 
great importance. It is different when milk coagulates 
spontaneously from when coagulated through the addition 
of acid or rennin. When cheese is to be made it is required 
that the casein gel shall have a definite solidity, on the one 
hand, and yet a definite plasticity on the other, otherwise a 
right development and distribution of the gas bubbles may 
not occur and so the ‘‘holes”’ be not right, as in Swiss cheese, 
for example. Proper measurement of such mechanical prop- 
erties belongs to the interesting field of gel physics. 

When casein is coagulated by an acid or, better, by rennin, 
the whole system goes solid but the process does not stop 
there. The gel contracts and squeezes off a greenish-yellow 
“whey.” Obviously, this is nothing but a vivid example 
of syneresis. 

The colloid chemist can apply his science to many prob- 
lems in bread making. In his terms, flour is made up of 


TECHNICAL APPLICATIONS 241 


three dry hydrogels, plant protein, starch and cellulose. 
The coarsely dispersed powder contains salts, sugar, acids 
and water in molecularly dispersed form. Dough is a gel 
which colloid-chemically has a structure similar to milk, 
containing in molecularly dispersed form besides dextrin, 
sugar and alcohol, various salts, acids and gases; in colloid 
form, the ‘“‘dissolved’’ plant proteins; in coarsely dispersed 
form, the swollen starch granules, gas bubbles, yeasts, lac- 
tic acid bacteria, etc. 

It has the properties of both liquids and solids. Like 
any liquid it assumes the shape of its container even though 
slowly and gas bubbles arising in the mass take on a spher- 
ical or lens-shaped form. On the other hand, it can be cut 
in pieces like a solid body and if suddenly subjected to pull, 
it tears exposing irregular surfaces. Such combination of 
liquid with solid properties is characteristic of gels and sol- 
vated emulsoids. 

What happens when bread is baked? Chemically, baked 
- bread is not unlike dough, but colloid-chemically the differ- 
ence is great. When baked the coarsely dispersed starch 
granules become highly dispersed and highly hydrated. 
The colloid albumins, on the other hand, are coagulated. 
Of great importance is the behavior of the gases produced 
when dough rises or is baked. In the process of baking 
the bubbles must not coalesce into large ones but must be 
fixed in a still finely divided state, just as solid reaction 
products may be kept in a state of high dispersion through 
the presence of a protective colloid. Only in this way will 
we obtain the coherent sponge of hydrated starch and coag- 
ulated protein permeated by innumerable small gas cham- 
bers which characterizes well baked bread. When the gas 
bubbles form too rapidly or come out in too gross form, 
holes or cracks appear in the bread thus giving rise to the 
commonest of baking failures. Well-made bread is a gel 
sponge or foam, analogous in its dispersoid-chemical struc- 
ture to pumice or whipped cream, the mechanical proper- 


242 COLLOID CHEMISTRY 


ties of the bread gel standing about half way between these 
extremes. 

The properties of flour which make it yield a good bread 
are known as its baking qualities. They are much discussed 
in practice for we seem to have no laboratory method of 
telling us whether a flour will bake well or not. In order 
to discover this a sample loaf must still be baked. As in 
the days of the ancients we do not discover metal in stones 
by chemical analysis but by fusing them. Can colloid chem- 
istry help us in this flour problem, for it seems proved! that 
physico-chemical differences in the several constituents of 
flour are responsible for its baking qualities? As in other 
colloid industries (as those of rubber and artificial silk) it 
has been found? that the viscosity of a diluted flour gel 
parallels the physico-chemical properties of the concen- 
trated gel. In keeping with the fact that the highly milled 
flour of war times yielded a poorer, heavier bread than the 
less milled flour of peace times, the viscosities of their “‘solu- 
tions” are also markedly different. The different baking 
values of wheat and rye flours are paralleled by correspond- 
ing viscosity differences. A medium concentration of acid 
in the dough is best for baking purposes and what this is 
may also be discovered by viscosimetric means. Flours 
which bake poorly are those which have a low ‘‘viscosity 
number.” How sensitive is the method may be seen from 
the fact that hard or soft waters which influence markedly 
the baking qualities of a flour betray themselves in vis- 
cosimetric tests. 

The “going stale” of bread is a familiar change which 
has not as yet been satisfactorily explained. It is most 
marked in white bread which becomes crumbly. The change 
is not due to mere water loss. Even in a tin box, in a sealed 
glass tube, in an atmosphere saturated with water vapor, 
bread goes stale. Chemical differences can not be dis- 


1 See, for example, M. P. Neumann, Brot u. Brotgetreide, 284, Berlin, 1914; — 
A. Maurizio, Nahrungsmittel aus Getreide, 1, Berlin, 1917. 

2 See the papers of Wotraana OstwaLp and H. Liturs on the colloid 
chemistry of bread in the Kolloid-Zeitschrift for 1919 and 1920. 


TECHNICAL APPLICATIONS 243° 


covered between fresh and stale bread. ‘The colloid chem- 
ist, however, notes marked differences in swelling capacity 
and observes that the starch constituent.is the fraction 
particularly affected. The process is at least partially re- 
versible for when bread is warmed over its freshness may 
be restored. Furthermore, bread goes stale faster at 0° C. 
than at higher or lower temperatures. Taken all together 
the change has much in common with that observed when > 
milk is coagulated and separates into clot and whey. Such 
syneresis may be seen even in ordinary starch paste. What 
must be the consequence of the syneresis? The sponge 
constituting the bread must suffer tension and its weaker 
lamelle be torn. This is why the bread becomes crumbly. 
The squeezing off of the fluid is best seen in the first periods 
of going stale. Bread has then a moist feel not limited to 
the dextrin-rich crust but discoverable in the body of the 
bread. Colloid chemistry seems capable therefore of con- 
-tributing to an understanding of this old and interesting 
problem also.! 

Let us now consider meat. Histologically, lean meat 
consists of muscle fibers and connective tissue, biochem- 
ically of muscle proteins and collagen. The muscle pro- 
teins are typical colloids and collagen is not only the source 
of gelatin but its Greek root, xod\d\a, is the baptismal 
source of the whole science of colloid chemistry. Fresh 
meat is notably tough and difficult of digestion. Only 
after it has been ‘‘hung”’ does it become tender and less 
difficult of digestion. What are the changes which the 
meat has suffered? Shortly after an animal has been killed, 
its fleshy portions — the muscles — go into “rigor mortis”’ 
which, some twenty-four hours later, passes off. Only then 
does meat assume that degree of tenderness, juiciness and 
palatability which we cherish. 


1 The papers of J. R. Karz [Zeitschr. f. Elektrochem., 19, 663 (1913); 
Zeitschr. f. physiol. Chem., 95, 104 (1915); 96, 288, 314 (1916)] should re- 
ceive special mention. Katz however attributes the going stale of bread to 
more complicated circumstances and uses colloid-chemical concepts like 
that of syneresis not at all or only as factors of secondary importance. 


244 COLLOID CHEMISTRY 


The onset and disappearance of rigor mortis are depend- 
ent upon a series of colloid-chemical changes in the muscle 
colloids. Shortly after death, various acids are formed in 
the muscle, particularly lactic acid. These make the fibrils 
of the muscle swell, the swelling expressing itself in that 
contraction of the muscle which characterizes the death 
rigor. Some authors, like O. von Furr and E. LEnx, 
believe that the acids are also responsible for the secondary 
softening of the muscle acting this time upon another col- 
loid, myosin. This they suppose to coagulate later than 
the muscle fibrils or at a higher concentration of the acid. 
I think it not impossible that the relaxation of the muscle 
—an internal shrinkage — may also be but another illus- 
tration of that spontaneous change so often discussed as 
syneresis. ‘The various theories! cannot be discussed fur- 
ther but it must be apparent that colloid-chemical ways 
— too many perhaps — of explaining these important prac- 
tical questions are at hand. 

The meats of different animals can be distinguished from 
each other by the amounts they swell in dilute salt solutions 
and it can also be determined in this way whether the meat 
has been freshly killed or hung, whether it has been frozen 
or dried, etc. Swelling capacity and degree of digestibility 
largely parallel each other. Veal, for instance, is not only 
easily digested but swells greatly. According to G. JuNG 
meat retains a greater water absorption capacity when 
salted dry than when pickled in brine. Pork can be dis- 
tinguished from other meats by the fact that its water hold- 
ing capacity suffers the least amount of change when cooked 
or dried. The effect of sour milk or vinegar upon meat, 
as in the preparation of ‘‘Sauerbraten”’ is a colloid-chemical 
one. The meat swells in these solutions — the connective 
tissue more than the muscle — becoming in consequence 
richer in water, more tender and more digestible. ‘The 


1 See, for example, the bibliographies in the dissertations of AMHERDT and 
JunG from the laboratory of Prof. W. Frei in Ziirich. 


TECHNICAL APPLICATIONS 245 


effects of boiling or roasting meat may also be analyzed 
colloid-chemically. 


818. 


These references to milk, bread and meat do not, of course, 
exhaust the applications of colloid chemistry to the problem 
of the preparation of our food stuffs. A colloid-chemical 
principle is utilized when gelatin is added to ice cream. It 
serves as a protective colloid for the fine crystals of ice which 
are formed in the process of freezing, giving these ice crys- 
tals a high degree of dispersion and thus the finished prod- 
uct its wished-for body and smoothness.! 

Colloid-chemical processes are also encountered in the 
production of mayonnaise? and of sauces of various kinds. 
Adsorption phenomena are called into service when the 
salt content of a bouillon is decreased so much that the 
effect may be recognized even by the sense of taste, simply 
by adding unsalted rice to the bouillon. It is also seen 
when the good housewife reduces the salt content of a too 
salty soup by stirring into it an egg. The art of cooking 
is a colloid-chemical art. 

But coffee and tea are also colloid solutions. The 
former lends itself splendidly to demonstration experi- 
ments in diffusion, dialysis, electrophoresis, ultramicros- 
copy, coagulation, adsorption, etc. Wine and beer, too, 
are colloid solutions.? The colloids of beer are positively 
charged and their presence gives beer its body and its 
foaming qualities. A beer problem of a distinctly colloid- 
chemical nature has proved of interest to Americans. 
Since they are accustomed to consume their beer at a 
temperature lying near the freezing point, the beer was 
found often to become turbid. This is due to the fact 

1 See J. ALEXANDER, Koll.-Zeitschr., 5, 101 (1909); 6, 197 (1910). 

2 See Martin H. Fiscuer and Marian O. Hooker, Fats and Fatty De- 
generation, New York, 1917. 

3 Many original papers or abstracts of the work of F. EmMsnuanprr, A. 


ReicHarpt, H, Luers, W. Winpiscu, W. Drerrics, etc., on the colloid 
chemistry of beer appear in the Kolloid-Zeitschrift. 


246 COLLOID CHEMISTRY 


that the colloid proteins of beer tend to precipitate at these 
low temperatures. By adding hydrating substances to the 
beer like lactic acid or traces of proteolytic ferments, it 
was found possible to hydrate the proteins so heavily that 
they no longer settled out. This trick must impress every 
colloid chemist as highly amusing, for to accomplish it 
the very substances are used which in the body are respon- 
sible for the production of edema. The Americans liter- 
ally consume beer which has been rendered ‘‘edematous.”’ 

Colloid-chemical principles are utilized in the refining of 
sugar. In this the sugar is separated from its colloid accom- 
paniments by processes of diffusion, dialysis, etc. We are 
face to face here with technical questions through the solu- 
tion of which I was told in America fortunes may be made. 
Various cane sugar molasses containing great quantities of 
sugar are sold as cattle feed, simply because the sugar can- 
not be separated from its colloid accompaniments. We 
seem to deal in these instances with adsorption compounds 
between pectin-like substances and sugar, and the colloid- 
chemical problem involved is that of the destruction of this 
combination.! | 

In the official testing of foodstuffs, colloid-chemical 
methods are used for the discovery of adulterants. I have 
already touched upon Ley’s silver test for the distinction 
of natural from artificial honey. <A colloid-chemical method 
for discovering the addition of agar to fruit jellies and 
marmalades makes use of the influence which such addi- 
tion has upon the form and the structure of LIESEGANG 
rings when formed in such jellies.2. The principles of emul- 
sification and of stabilization of the emulsions are of im- 
portance in the manufacture of oleomargarine. 

Perhaps I have exhausted your patience by this endless 
recital. But is it my fault that relations are so intimate 


1 See, for example, the newer studies of Prrx, Intern. Sugar Jour., 21, 70 
(1919) and F. W. Zersan, Jour. Ind. Eng. Chem., 12, 744 (1920). 
2 See the paper of KE. Marriaae, Koll.-Zeitschr., 11, 1 (1912). 


TECHNICAL APPLICATIONS 247 


that every colloid chemist needs to greet every good cook 
as a colleague? | 

But wherein lies the progress to be expected of a conscious 
application of colloid chemistry to food chemistry? We 
have made butter and cheese and baked bread for ages past 
without any knowledge of colloid chemistry. Colloid-chem- 
ical treatment of these questions brings new terms but does 
it also bring us new things and not merely a rehash of the 
old? 

It must be admitted that even colloid chemistry can not 
make caviar of turnips and that it can not be a patent med- 
icine cure for every difficulty of industrial practice. Never- 
theless, colloid chemistry does produce practical results. 
One may buy fresh breakfast rolls in the cities of Holland, 
for example, in spite of the fact that night baking is for- 
bidden. J. R. Katz has shown how rolls may be baked one 
day and still be fresh the next. But the colloid chemist 
would be satisfied without such practical results if he could 
contribute to a better understanding of what is already 
known, for to understand better the nature of what has been 
done is to know better what may be done. 

Every new science has to produce its new terminology. 
But when flour is defined as a mixture of three hydrogels 
of which two (protein and starch, are lyophilic, something 
more is given than new words. It is made clear that the con- 
_ stituents of flour are capable of swelling and that this swelling 
is greater for starch and protein than for cellulose. It 
follows that the laws which colloid chemistry has discovered 
for the behavior of lyophilic substances hold for the constit- 
uents of flour. But whoever makes such a new definition 
must assume, too, full responsibility for the associated truths, 
It is serious work therefore when even old and familiar 
phenomena are redefined! colloid-chemically. With this I 
do not wish to deny that the thing has, at times, been done 

1 An illustration taken from another field may serve to clarify the situation. 
It was known even before the theory of electrolytic dissociation was advanced 


that silver nitrate would react with ‘“‘dissolved ” chlorine compounds. Today 
we say that silver nitrate is a reagent for ‘‘chlorions.”” No one will deny the 


248 COLLOID CHEMISTRY 


superficially and with little conscience, but if such errors — 
hardly to be avoided in any rapidly growing science — 
are overlooked, the fact remains that colloid chemistry is 
of inestimable theoretical and practical value to the food 
chemist. That much remains to be done should be occa- 
sion for joy. Let us at it. 


$19. 


Colloid chemistry has something to say about fuels. 
Why has fresh coke a greater (maximal) capacity for hold- 
ing water than aged coke? It seems to me that this is 
another case of syneresis. As H. WiInTER has recently 
shown!, coke and hard coal must obviously be the carriers — 
of highly dispersed systems similar to gels when the origin 
of these substances and their relation to soft coal, lignite, 
peat, wood, etc., is remembered. 

Why does it not pay to squeeze the water from peat by 
mechanical methods? It is because it is bound to the peat 
as hydration water and it requires many atmospheres pres- 
sure to bring about separation. This is why all rational 
present day or future methods must start with the separa- 
tion of this colloidally bound water through the addition 
of coagulating chemicals, through the destruction of the 
gels by autoclaving at 180° C. or thereabouts (the EKEBERG 
process) or through the plentiful use of time, for peat also 
shows in beautiful fashion the phenomenon of syneresis, 
that is to say, a spontaneous giving off of its dispersion me- 
dium with time. Peat spread out during the summer months 
dries out greatly in spite of the fact that it is frequently 
rained upon. In this instance, of course time means 
money.” 
progress represented by such a “restatement of the fact in other words” 
with all its consequences as now embraced in our concept of the “ion.” 

1 H. Winter, Koll.-Zeitschr., 19, 8 (1916). 

2 Hay also shows syneresis as demonstrated by the fact that it ‘‘sweats”’ 
after having been dried superficially. For a discussion of this question and 


its réle in the spontaneous combustion of hay, etc., see G. Laupprr, Land- 
wirtschaftl. Jahrb. d. Schweiz, 1920 (reprint). 


TECHNICAL APPLICATIONS 249 


When raw fuels, in order to save their valuable by-prod- 
ucts, are first subjected to coking or fractional distillation, 
limitless dispersed systems like suspensions, emulsions and 
foams are encountered. The production, destruction or 
conservation of these belongs to the most important as well 
as most difficult of the problems of these industries. 

The mineral oil industry is permeated with colloid prab~ 
lems. As D. Houps! has shown, crude oil and some of its 
various fractions contain materials which, like the asphalts, 
are present in typically colloid subdivision. The funda- 
mental process of dehydrating crude oil represents nothing 
but the coagulation of a water-in-oil emulsion; and its 
purification through the addition of solid adsorbents, like 
Florida earth, or by means of electrophoresis, is nothing 
but a giant colloid reaction. When liquid fuels are burned 
in motors, etc., the attempt is made to disperse them as 
highly as possible through a suitable dispersion apparatus 
(the carburetor). ‘‘Colloid coal’? has been much discussed 
lately. The name is given to dispersions of coal dust in 
fuel oil, the suspensions produced actually being so fine that 
the coal particles no longer settle out. It has been 
suggested that coal dust might in this way be used in DrgesEL 
motors but the subject is too new to be ready for dis- 
cussion. : 


§20. 


Fuels and raw materials enter a factory; finished articles 
and waste products, like waste liquors, sludges, smoke and dust 
depart from it. To dispose of smoke and dust the methods 
of dispersoid chemistry must be employed. A hundred 
years ago HoHLFELD, teacher of mathematics in the Leip- 
zig Thomas School,? suggested that the smoke nuisance 
might be met through the use of electrophoresis discovered 


1 D. Houpsg, Koll.-Zeitschr., 3, 270 (1908). 
2 See in this connection the interesting lecture of V. KOHLSCHUTTER, 
Nebel, Rauch und Staub, Bern, 1918. 


250 COLLOID CHEMISTRY 


by Revss in 1809. This idea, developed in our time 
through the work of the ‘American, F. CoTTRELL, is used 
today to this very end. But not only is dust ‘‘destroyed”’ 
in such fashion but the same method is used in order to 
recover in ponderable form valuable materials which might 
otherwise be lost, as potassium from the dust of cement 
ovens. 

When I tell you finally, that a large part of the refuse 
materials found in our drainage systems (constituting in 
city sewers, for example, fifty to sixty per cent of the solids 
contained in the waters flowing through them) is found 
here in a colloid state and that colloid-chemical methods 
must in consequence be used to handle them, you will 
perhaps be inclined to agree with me when I say that 
things not only begin in colloid chemistry, but in colloid 
chemistry they end. 


§21. 


With this I, too, must conclude not only my remarks upon 
the technical applications of colloid chemistry but the entire 
series of my lectures. I shall be satisfied if I have suc- 
ceeded in making clear to you the freshness of the points 
of view of colloid chemistry, its wealth, and its inexhaustible 
possibilities of scientific and practical application. It is 
these which I think justify us in looking upon colloid chem- 
istry as entitled to independent existence. 

In retrospect you will, perhaps, be tempted to ask me the 
following questions. If it is true that we are dealing with 
a science so rich in ideas and so pregnant with possibilities 
of scientific application — I presume too much, perhaps, 
when I assume that my lectures have given you this im- 
pression — if all this is true, why is it that colloid chemistry 
has not long been known as an independent science? Why 
is it that this science which deals in such large measure with 
commonplace and everyday sorts of things has been studied 
systematically for but a few years? 

I think that the answers to these questions are about as 


TECHNICAL APPLICATIONS 251 


follows. Physics has until recently busied itself chiefly with 
the properties of matter in mass; chemistry, on the other 
hand, has dealt chiefly with the smallest particles of matter 
such as atoms and molecules. Relatively speaking, we 
know much of the properties of large masses and we talk 
much, also, of the properties of molecules and atoms. It 
is because of this that we have been led to regard everything 
about us either from the standpoint of physical theory or 
from that of molecular or atomic theory. We have entirely 
overlooked the fact that between matter in mass and matter 
in molecular form there exists a realm in which a whole 
world of remarkable phenomena occur, governed neither by 
the laws controlling the behavior of matter in mass nor yet 
those which govern materials possessed of molecular dimen- 
sions. We did not know that this middle country existed, 
how large a number of natural phenomena belonged in it, 
nor how greatly the degree of dispersion determined their 
behavior. We have only recently come to learn that every 
structure assumes special properties and a special behavior 
when its particles are so small that they can no longer be 
recognized microscopically while they are still too large to 
be called molecules. Only now has the true significance of 
this region of the colloid dimensions —’THE WoRLD OF 
NEGLECTED DIMENSIONS — become manifest to us. 

If some of my explanations have seemed not clear or 
inadequate, I beg you to consider this not as characteristic 
of colloid chemistry, but as dependent solely upon my 
personal shortcomings. A science may attain to fullness; 
her disciples, never. 











AUTHOR INDEX 


A. 


Acueson, 5, 196, 197. 
ALEXANDER, J., 245. 
AMANN, J., 19. 
AmBRONN, H., 208. 
AMHERDT, 244. 
ARNOLD, H., 204. 
ARRHENIUS, SVANTE, 154. 
ATTERBERG, A., 162. 
AUERBACH, R., 66, 201. 
Avocapro, 151, 152. 
AXELROD, S., 232. 


B. 


BacHMann, 97, 110. 
BacHMETIJEW, 78. 

BaRKLA, C., 57. 

Bauer, H., 189. 

Bayer, FRIEDRICH, 234. 
Bayuiss, W. M., 166. 
BEcHHOLD, H., 48, 78, 165, 188. 
BEIJERINCK, N., 168. 
BELTZNER, J. G., 227. 


BEMMELEN, J. M. van, 132, 141, 164. 
BENEDICKS, C., 35, 215, 217, 218, 219. 


BERMAN, 43. 

BERNSTEIN, G., 232, 233. 
Bittz, W., 132. 

Brnz, 189. 

BircueEr, E., 187. 

Boss, N., 148. 

Borazzi, F., 52, 166. 
Braag, 57. 

Brepie, G., 34, 79. 
Breum, H., 162. 

Brown, 44, 45, 47, 60, 62, 151, 167. 
Bruyn, Losry DE, 54. 
ButiowA, 165. 
BuNScHOTEN, B. E., 233. 


255 


Buitscu it, O., 100, 107, 110, 149, 168, 
175. 
Bysow, B., 227. 


C. 


Cavazzi, A., 209. 
CHAMBERS, R., 174. 
Curarl, R., 171. 
CHRISTIANSEN, 6, 61. 
Comte, A., 135. 
CorRNALBA, 238. 
Cornu, F., 157, 161. 
CoTTrRELL, F., 250. 
CzocHRALsKI, J., 219. 


D. 


Der Broyn, Losry, 54. 
Depye, P., 56, 57. 
DECKHUYZEN, 152. 
DimsE1, 249. 
Dietricn, W., 245. 
Drrmar, D., 227. 
DoeEtTER, C., 156. 
Doute, W., 91. 
Donat, J., 138, 189. 


BK. 


EBNER, V. von, 149: 

Eber, R., 188. 

Ea@certz, 216, 217. 

EHRENBERG, P., 40, 139, 162, 165. 
EXRRENHAFT, F., 154, 155. 
EKEBERG, 248. 

EMSLANDER, F., 245. 


¥. 


Farapay, 14, 23, 50, 75. 
FARADAY SOCIETY, 89. 


256 


Ficuter, F., 73, 74. 

Fick, A., 166. 

Fiscuer, A., 181. 

Fiscoer, Martin H., 6, 46, 76, 78, 
143, 166, 171, 174, 176, 178, 1883, 
184, 185, 187, 235, 245. 

FLEISCHMANN, W., 240. 

FREI, W., 244. 

FrREUNDLuIcH, H., 74, 89. 

FRIEDENTHAL, H., 188. 

Firru, O. von, 178, 244. 

Kurta, R., 55. 


G. 


GarpuxKow, N., 107, 166, 167. 

GARRAND, J. D., 74. 

Gaza, W. von, 188. 

Gspss, WILLARD, 87, 130, 214. 

Gigs, W., 183. 

GLOVEZYNSKI, T., 76. 

Gop.LEwskI, T., 150. 

GoxtpscuHmiIpT, F., 235. 

GrauaM, Tuomas, 3, 5, 8, 9, 10, 11, 
70, 88, 98, 99. 

Gtrriter, W., 214, 216, 217, 218, 
219, 220, 221. 

Gurwitscy, L., 125. 


H. 


Hater, R., 145, 221. 

Hanpbovsky, H., 120. 

Harpy, W. B., 100 

Harriss, C., 229, 232. 

. Harscuex, E., 92, 112, 113, 114. 

Hauser, O., 141. 

Hevesy, G. von, 74. 

HEYDEN, VON, 199. 

Heyn, 220. 

HILGARD, E., 162. 

HinricHsENn, F. W., 227. 

Hoser, R., 165. 

Horr vAn’T, 42, 145, 214. 

HoFrrMann, J., 201. 

HorMeistER, F., 
209. 

HOHLFELD, 249. 


121, 1-168, 72) 


AUTHOR INDEX 


Ho.pe, D., 249. 
Hooker, Martian, O. 245. 


IsuizaKA, N., 89. 
ITERSEN, G. VAN, 229. 


J. 


June, G., 244. 
Jiprner, H. von, 217. 


K. 


Kanero, W., 54. 
Karczaa, L., 6. 
Katz, J. R., 248, 247. 
KauFFMANN, M., 189. 


-KEISERMANN, 8., 206. 


KeEwuER, R., 74, 145. 
Kircunor, F., 232. 
Kircuner, W., 240. 
Kirn, G. L., 174. 
KoxHLscHuTtTer, V., 213, 249. 
Konowatow, D., 95. 
Kossonocow, J. J., 76. 
Kratz, G. D., 234. 
Kuster, E., 112, 160. 
KuZxEu, 199. 
Kyroprontos, 8., 111. 


L. 


Lance, 187. 

LANGE, O., 202. 

Lave, M., 56, 57. 

LauppER, G:, 248. 

Lea, Carey, 11, 34, 65. 

Le Buanc, M., 54, 55. 

LEIMDORFER, J., 235. 

Lenk, E., 244. 

Lepxowsky, W. VON, 95. 

Ley, 139, 246. 

LrypDE, 220. 

Liesic, Justus von, 182, 236. 

LiEsEGANG, R. E., 64, 112, 118, 114, 
115, 116, 159, 181, 203, 217, 246. 


AUTHOR INDEX 257 


Liner, 8. E., 47. 156, 174, 177, 178, 209, 211, 221, 
Luoyp, JoHn Urt, 127. 227, 242. 
Lors, JAcquss, 174. OswaLp, A., 187. 
LorEweE, 8., 188. 
Lorenz, R., 220. Pp 
LotrerMoseER, A., 74, 123, 151. 
Lupwie, C., 49, 166. PAAE,. Gi 5, O: 
Ltrs, H., 145, 242, 245. Paine, H., 89. 
Lipro-Cramer, 64, 65, 140, 203. Panera, F., 150. 
Pau, THEODOR, 236. 
M. Paul, Woureane, 120, 121, 166, 
171, 178, 188. 
MatFitTano, G., 10, 48. Pawtow, P., 79, 154. 
Mann, G., 181. PEEK, 246. 
MarriaGE, H., 246. PELET-JOLIVET, J., 221. 
Martin, C. J., 48. PERNTER, J. M., 153. 
MarTu.a, J., 235. PERRIN, J., 79, 151. 
Maurizio, A., 242. . Picron, H., 47. 
Mayer, 127. PrERonI, A., 79. 
MeEcKLENBURG, W., 141. Popszus, E., 204. 
MEIssNn_ER, F., 79. Popper, 199. 
Mewz, W., 97. PosnJAk, G., 233. 
Micuag.is, W., 158, 206, 207, 208. 
Mor.uENporFF, W. von, 170, 180. . Q 
Mo.uer, W., 221. 
Mier, E., 35, 212. QuINCKE, G., 68, 73, 107. 
Munov.er, K., 55, 146. 
R. 
ae Rarro, M., 79. 
NAUMANN, 202. Ramsay, W., 47. 
NavassaRT, E., 70. RayLeEIGcH, Lorp, 55, 62. 
NevBeERrt, J. K., 204, 205. ReIcHarvt, A., 245. 
Neumann, M. P., 242. Revers, W., 140. 
NEuNER, Cur., 221. ) Reuss, 250. 
RHUMBLER, L., 165, 169. 
RicuTEerR, Bens. JEREMIAS, 3, 23. 
_ ’ Rivne, F., 57. 
Ovan, 8., 41, 124, 162. Rouwanp, P., 206. 
OxrsPER, 143. RontaEn, 17, 56, 57. 
Oxton, E., 124. RossEM, A. VON, 229. 
OsTROMYSLENSKY, 233. Ruwuand, W., 180. 
OstwaLp, WALTHER, 234. RuszNyak, Sr., 80. 
OsTWALD, WILHELM, 114, 135. 
OstwaLp, Wo.tFreana, 5, 6, 24, 35, S. 


41, 46, 48, 53, 55, 56, 65, 66, 71, 
74, 75, 76, 78, 97, 100, 102, 103, Sasatinr, J., 147. 
106, 119, 141, 143, 145, 146, 154, Sansom, N., 73, 74. 


258 


Samec, M., 102. 

Sanpavist, H., 7, 149. 

SanIn, A., 132. 

Scuave, H., 188. 

ScHaPer, A., 177. 

ScHERRER, W., 56, 57. 

ScHIpRowITz, P., 227. 

_ Scumauss, A., 154. 

SCHULEMANN, W., 180. 

SCHWARZSCHILD, K., 154. 

SHEKERA, F., 150. 

SELMI, F., 3. 

SIEDENTOPF, H., 58, 1438, 155. 

SmoLucHowsk], M. von, 55. 

SPEK, J., 176. 

SPENCE, D., 227, 228, 229, 238, 234. 

SPRING, W., 54, 58, 235. 

Stas, G., 76. 

StTrasny, E., 221. 

STIEGLITZ, J., 34. 

STODEL, G., 188. 

STRIETMANN, W. H., 178. 

SusvukI, 162. 

SVEDBERG, THE, 6, 22, 35, 47, 63, 
67, 68, 69, 71, 149. 


ik 


Tuomas, A. W., 74. 
TuHomson, J. J., 130. 


AUTHOR INDEX 


TRAUBE, J., 189, 209. 
Tynpat1, J., 50, 51, 52, 58, 54, 55, 
56, 57, 58, 59, 61, 145. 


ve 


Van’? Horr, 42, 145, 214. 
Voit, K. von, 236. 


YW, 


WALDEN, P., 7. 

WEBER, C. O., 52. 

WEGELIN, G., 35. 

WerImarw, P. P. von, 19, 26, 27, 
49, 81, 201, 215, 216. 

WENZEL, 79. 

WIEGNER, G., 165, 237, 238. 

Winpiscu, W., 245. 

Winter, H., 248. 

WINTERSTEI, 52. 

WoHLER, L., 69, 182. 

Wo tsk, P., 55, 56, 209. 


Z. 


ZANKER, W., 202. 

ZERBAN, F.. W., 246. 

ZIRKEL, 202. 

ZsSIGMONDY, R.., 14, 58, 59, 60, 97, 110. 


SUBJECT INDEX 


A. 


Acacta, 103. 

Acetyl cellulose ester, 227. 

Acheson graphite, 197. 

Acids and swelling, 105; and edema, 
183; and cement, 208. 

Adulterants in honey, 139. 

Adsorption, 87, 124; and chemical 
consequences, 125, 164; of alka- 
loids, 127; and Gibbs’ theorem, 
130; mutual, 131; in soil, 164; 
therapy, 188; in dyeing and tan- 
ning, 221 to 223; in vulcanization, 
229. 

Adsorption isotherm, 128. 

Agar, 98. 

Albumin (see also, Protein) 88, 98. 

Alkali blue, 72. 

Alkalies and swelling, 105; and 
liquid ammonia, 141; and clays, 
203. 

Alkaloids, 127. 

Allotrophy, 216. 

Alloys, 213, 221. 

Aluminium silicate, 204. 

Ammonium sulphate, 88. 

Analysis, colloid, 8. 

Aniline dyes, 63. 

Animal membranes, 9. 

Anthrapurpurin, 206. 

Antipyrin, 78. 

Applications of colloid chemistry to 
analytical chemistry, 137; to or- 
ganic chemistry, 141; to dyeing, 
143, 221 to 223; to physical 
chemistry, 145; to catalysis, 147; 
to crystalline liquids, 148; to 
radioactivity, 150; to Avogadro’s 
constant, 151; to cosmic physics, 
152; to mineralogy, 155 to 160; 


259 


to geology, 161; to soil chemistry, 
161 to 165; to biology 161 to 
178; to morphology, 174; to 
growth, 177; to muscular con- 
traction, 178; to secretion, 179; 
to vital staining, 180; to syn- 
thetic biochemistry, 182; to path- 
ology and medicine, 183 to 190; 
to technology, 193 to 251; to 
lubricants, 196; to light filaments, 
198; to glass, 200; to ultramarine, 
201; to photography, 203; to 
ceramics, 204; to hydraulic ce- 
ments, 205 to 209; to metallurgy, 
209; to alloys, 213 to 215; to 
steel, 216 to 221; to dyeing and 
tanning, 143, 221 to 223; to cel- 
lulose industries, 225; to silk 
manufacture, 226, to rubber, 227; 
to vulcanization, 229 to 234; to 
soap manufacture, 235; to food 
chemistry, 236 to 248. 

Aquadag, 196. 

Artificial silk, 226. 

Arsenic trisulphid, 47. 

Astrospheres, 174. 

Austenite, 217. 

Avogadro’s constant, 151. 


B. 


BAKELITE, 227. 

Barium sulphate, 27. 

Beer, 245. 

Benzol, 122. 

Berlin blue, 25. 

Bile salts, 188. 

Biology, 162 to 182; 
182. 

Blue rock salt, 155. 

Borax beads, 138. 


synthetic, 


260 SUBJECT INDEX 


Bread, 240; baking of, 241; going 
stale of, 242. 

Bricks, 195, 197. 

Brownian movement, 44, 45, 151. 


G> 


CatciuM CARBONATE, 114. 

Calcium sulphate, 209. 

Capillary analysis, 72. 

Capillary phenomena, 77. 

Carbon, 125, 126, 216, 219, 220. 

Cassius purple, 137. 

Catalysis, 146, 147, 148. 

Catalytic effects, 79. 

Cell, osmotic processes in, 170; di- 
vision of, 175. 

Cellon, 227. 

Cellulose, 225. 

Cement, 205 to 209. 

Cementite, 216. 

Ceramics, 204. 

Chamberland filter, 47. 

Chamois, 225. 

Changes in state, 93. 

Christiansen colors, 6, 61. 

Chromisomers, 141. 

Chromium, 200. 

Cinnamic ethyl ester, 61. 

Classification of colloids, 39, 40, 41, 
42. 

Clays, 203, 204. 

Clay sphere, 77. 

Clothes, 193. 

Clouds, 153. 

Cloudy swelling, 176. 

Coagulation, 86, 89; of colloids, 118, 
119, 120, 121, 122. 

Collodion capsules, 11, 48. 

Collodion filters, 48. 

Colloids, definition of, 4, 35, 80; 
concept of, 5; examples of, 5; 
analysis of, 8; diffusion of, 8; 
membranes of, 8; and relation to 
suspensions, 9, 14, 20, 21, 35; 
and relation to molecular solu- 
tions, 14, 19, 21, 35; preparation 
of, 23, 27, 34; of gold, 23; classi- 


fication of, 39, 42; granular, 40; 
properties of, 44; Brownian move- 
ment in, 44; diffusion and dialy- 
sis of, 45; salts and hydrated, 46; 
filtration of, 48; optical proper- 
ties of, 50; Tyndall cone in, 
51; hydrated, 52; ultramicroscopy 
of, 59; of sodium chlorid, 61; 
and maximum properties, 62; 
colors of, 61, 62, 63; law of 
color in, 64, 65; and transition 
phenomena, 67; and electrical be- 
havior, 71; and velocity of mi- 
gration, 74; and freezing and 
melting points, 76, 77, 78; and 
capillarity, 77; and reaction ve- 
locity, 79; internal changes in, 
85; adsorption in, 87; instability 
of, 88; and kinetic treatment, 88; 
viscosity of, 89; and temperature, 
91; and time factor, 92; and 
added substances, 92; and critical 
mixtures, 94; and foaming, 96; 
separation phenomena in, 97; 
and syneresis, 99, 106; and swell- 
ing, 101, 102, 103; effect of addi- 
tions to, 105; pressures and swell- 
ing of, 105; and structure, 106, 
107, 108; as gels, 109, 110; pe- 
riodic precipitations in, 112 to 115; 
frost figures in, 116; coagulation 
of, 118, 119, 120, 121, 122; pro- 
tective, 121; and radiant energy, 
122; and peptization, 123; and 
adsorption, 124 to 132; and Gibbs’ 
theorem, 130; importance of, 135; 
of noble metals, 137, 138, 139; 
and honey, 139; and analytical 
chemistry, 137; and organic chem- 
istry, 141; and dyeing, 148, 221, 
222, 223; and physical chemistry, 
145; solvation of, 146; and cat- 
alysis, 147; and crystalline li- 
quids, 148; and radioactivity, 150; 
and Avogadro’s constant, 151; _ 
and cosmic physics, 153; and 
mineralogy, 155 to 160; and geol- 
ogy, 161; and soil chemistry, 161 


SUBJECT INDEX 


to 165; and biology, 165 to 182; 


and medicine, 183 to 190; and 
technology, 193 to 251; and lu- 
bricants, 196; and incandescence, 
198; and glass, 200; and ultra- 
marine, 201; and pigments, 202; 
and photography, 203; and cer- 
amics, 204; and hydraulic ce- 
ments, 205 to 209; and metal- 
lurgy, 209; and alloys, 213 to 215; 
and steel, 216 to 221; and dyeing 
-and tanning, 148, 221, 222, 223; 
and cellulose industries, 225; and 
silk manufacture, 226; and rubber 
manufacture, 227 to 234; and 
soap manufacture, 235; and food 
chemistry, 236 to 248. 

Colloid chemistry, 3; history of, 4; 
of indicators, 144. 

Colloid coal, 249. 

Colloid filters, 48. 

Colloid ice, 6. 

Colloid ions, 75. 

Colloid mercury, 188. 

Colloid nickel, 188. 

Colloid pharmaceuticals, 188. 

Colloid silver, 34, 139, 188. 

Colloid state, 22, 36. 

Colloid sulphur, 188. 

Colloid water, 6. 

Color, of colloids, 61; and degree of 
dispersion, 66; and size of par- 
ticle, 66. 

Colors, Christiansen, 6, 61; inten- 
sity of, 63; of gold, 64; law gov- 
erning, 65; of metals, 68; of 
indigo, 69; of clothes, 193. 

Concentration, and degree of dis- 
persion, 26. 

Concentration function, 128. 

Condensation method, 23, 26. 

Conductivity in colloids, 75; in 
gases, 75; in electrolytes, 75. 

Congo red, 144. 

Congorubin, 144. 

Constant, Avogadro’s, 151. | 

Contraction, muscle, 178. 

Cooking, 239 to 245. 


261 


Cosmic physics, 152, 153, 155. 

Cotton, 225. 

Critical mixtures, 94. 

Critical temperature, 94. 

Crude oil, 249. 

Crystalline forces, 46. 

Crystalline liquids, 7, 147, 148, 149; 
as colloids, 149. 

Crystalloids, diffusion of, 8. 

Crystals, swelling of, 101; absorption 
of dyes by, 140, 141. 


D. 


DayLiGut, 152. 

Decimal division, 126. 

Degree of dispersion, 17, 18, 19, 43; 
in Berlin blue, 25; and diffusion, 
46. 

Dialysers, 9. 

Dialysis, 9, 44. 

Diffraction, 51, 58. 

Diffusion, 8, 44, 45, 47. 

Diffusion coefficient, 47. 

Dispersed systems, 16, 17, 18, 19, 20, 
21, 42; properties of, 43; me- 
chanical properties of, 43. 

Dispersoids, 16, 17, 18, 19, 21; of 
the heavens, 154. 

Dispersion degree, 43. 

Dispersion method, 23. 

Doughs, 108. 

Drainage, 250. 

Dust, 249; cosmic, 153. 

Dyeing, 221 to 223; fast, 223. 

Dyes, 143. 


E. 


Eprma, 183, 184, 185; and free acid, 
186. 

Egg white, 122. 

Electrical behavior of colloids, 71. 

Electrical dispersion, 34. 

Electrolytes and colloids, 92, 120; 
and swelling, 105; and peptization, 
123, 124. | 

Electrolytic metallurgy, 212. 


262 


Emulsification, 245. 
Emulsoids, 40, 86, 91, 95, 96;  co- 
agulation of, 120. 


BE 


Farapay’s Law, 75. 

Fast dyeing, 223. 

Ferrite, 217. 

Ferrocyanid, 25. 

Fertilization, 173. 

Fibrin, 105; and water absorption, 
171: 

Filter paper, 47; and colloid an- 
alysis, 72; freezing of, 77, 78. 

Filters, 12, 20, 47; colloid, 48; col- 
lodion, 48. 

Flea bites, 184. 

Flotation, 210; model of, 211. 

Flour, 240. 

Foams, 41, 96. 

Fog, 42. 

Food chemistry, and colloids, 236. 

Free acid and edema, 186. 

Freezing point, 76, 77, 78. 

Frog, developing, 177. 

Frog eggs, 169. 

Frost figures, 116. 

Fuels, 248. 

Fuller’s earth, 122, 125. 


G. 


GALALITE, 227. 

Gall stones, 188. 

Gas + liquid dispersoids, 41, 42. 

Gas + solid dispersoids, 42. 

Gastrulation, 176. 

Gelatin and color of colloids in, 64; 
and viscosity, 90, 91, 92, 98; syn- 
eresis in, 99; swelling of, 105; 
precipitations in, 112; and glass- 
chipping, 119; and water absorp- 
tion, 171; and artificial silk, 226. 

Gels, 86, 99, 108; as filtration mem- 
branes, 48; viscose, 88; kinetics 
of, 108; freezing of, 116. 

Geology, 161. 


SUBJECT INDEX 


Gibb’s rule, 87; in alloys, 214. 

Glanzgalvanisation, 212. 

Glass, chipping of, 119; ruby, 200; 
blue, 200. 

Glaucoma, 186 

Glue, 5, 101, 235; in cement, 208. 

Gold chlorid, 23. 

Gold, colloid, 23, 64, 65; colors of, 
64, 65; freezing of, 77; tests for, 
137; in glass, 200. 

Granular colloids, 40. 

Graphite, 73, 196. 

Gravity and dispersoids, 151. 

Gredag, 198. 

Growth, 177. 

Gum arabic, 86. 


1a F 


Hay, 248. 

Heterogeneous systems, 16, 17. 

Hide, 224. 

Homogeneous systems, 17. 

Honey, 139, 246. 

Humus acids, 139, 162. 

Hyaline minerals, 157. 

Hydrated colloids, 52, 90; coagula- 
tion of, 120. 

Hydrates, 141. 

Hydration, 52, 141; of protoplasm, 
169, 170, 171. 

Hydraulic cements, 205 to 209. 


ibs 


INCANDESCENT FILAMENTS, 198. 

Ice crystals, 116 

Indicators, 144. 

Indigo, 69. 

Inflammation, 187. 

Infusorial earth, 39. 

Ink, 194. 

Instability of colloids, 88. 

Intensity of colors, 63. 

Ions, 75; coagulating, 119; stabil- 
izing, 123, 124 

Iron alloys, 213, 

Iron ferrocyanid, 25. 


SUBJECT INDEX 


Tron hydroxid, 72, 74. 
Isobutyric acid-water, 97. 
Isochemites, 157. 
Isocolloids, 142, 231. 
Isopren, 142. 


K. 


Kinetics oF Gets, 108. 
Kolloid-Zeitschrift, 195. 


L. 


Latex, 227. 

Law, von Weimarn’s, 26; Ostwald’s, 
64, 65; Faraday’s, 75; Wenzel’s, 
79; Gibbs’, 87. 

Lead chromate, 112, 114. 

Leather, 193, 224. 

Ley’s test, 139. 

Liesegang rings, 112, 159. 

Life processes, 165. 

Light pressure, 154. 

Liquid + gas dispersoids, 42. 

Liquid + liquid dispersoids, 42, 86. 

Liquid + solid dispersoids, 41. 

Living matter, 165 to 168. 

Lloyd’s reagent, 127, 128. 

Localization, 172. 


M. 


MaRrTENSITE, 217. 

Mastic sol, 72. 

Maximum, and colloid realm, 62, 79; 
absorption, 65. 

Mayonnaise, 245. 

Meat, 243; hanging of, 243. 

Mechanical properties, 43. 

Melting point, 76. 

Membrane, 9. 

Mercerization, 224. 

Mercerized cotton, 226. 

Mercuric sulphid, 12. 

Metals, dispersion of, 34. 

Metallurgy, 209. 

Micro-dissection, 178. 

Milk, 237; coagulation of, 239. 


263 


Mineralogy, 155. 

Mining, 210. 

Molecular solutions, 14, 19, 20. 

Molecularly dispersed solutions and 
Tyndall cone, 54, 

Molecules, 12, 

Mortar, 205. 

Mucilage, 235. 

Muscular contraction, 178. 


N. 


Ne EDT 52: 

Neglected dimensions, world of, 251. 

Night blue, 72. 

Noble metals, recognition of, 137. 

Non-electrolytes and colloids, 92; 
and swelling, 105. 


O. 


OcHRES, 202. 

Oildag, 196. 

Oleomargarine, 246. 

Opal, 158, 159. 

Opalescence, 60, 95. 

Optical dust, 54. 

Optical properties, 50. 

Optical rotation, 70. 

Order of color change, 65. 

Organic chemistry and colloids, 141. 

Osmondite, 217. 

Osmotic pressure and water absorp- 
tion, 170. 

Osmotic processes in cell, 170. 


P, 


Paints, 202. 

Palladium hydroxid, 189. 
Parchment paper, 9, 225. 
Parthenogenesis, 174. 

Paste precipitates, 27. 

Pathology, 183. 

Peat, 248. 

Peptization, 86, 123; in clays, 204. 
Periodicity, 15, 18. 

Perlite, 216, 217. 


264 


Petroleum, 122. 

Pharmaceuticals, colloid in nature, 
188. 

Phases, 98. 

Phenol-water, 94, 96. 

Photographic plates, 64. 

Photography, 57, 140, 203; and 
Tyndall cone, 54. 

Photophoretic velocity, 154. 

Physical chemistry and colloids, 145. 

Pig bladder, 9, 50. 

Pigments, 202. 

Piperidin, 234. 

Plaster-of-Paris, 209; viscosity of, 
209. 

Platinum, colors of, 68; and hydro- 
gen peroxid, 79; tests for, 138. 

Points, 88. 

Pores, 20, 47. 

Potassium, swelling of, 101. 

Potato starch, 108. 

Precious stones, 156. 

Precipitates, 27, 28, 29, 30, 31, 32, 33; 
periodic, 112; and analytical chem- 
istry, 137. 

Pressure of swelling, 105. 

Protective colloids, 121. 

Protein, 88, 98, 105. 

Protoplasm, 165, 166, 167, 168. 

Pyrosols, 220. 


Q. 


QUARRYING, 19 
Quartz, solubility of, 158. 


R. 


Radioactive SuBsTANcEs, 147. 

Radio-chemistry, 136, 150. 

Reaction velocity, 79. 

Reagent, Lloyd’s, 127, 128; Mayer’s, 
127. 

Red gold, 23, 24, 200. 

Red ochre, 202. 

Refraction, 50, 51. 

Reichel filter, 47. 


SUBJECT INDEX 


Reversibility, 124. 

Rigor mortis, 243. 

Rings, Liesegang, 112 to 115, 159, 160. 

Rock salt, blue, 153. 

Réntgen rays, 17, 57; and silicic 
acid gels, 111. 

Réntgen ray Tyndall cone, 56. 

Rotation, optical, 70. 

Rubber, 227; swelling of, 101; syn- 
thesis of, 142, 230; in manufacture, 
227; vulcanization of, 229; as 
isocolloid, 231; milling of, 233; 
stabilization of, 234; and _ piperi- 
din, 234; regeneration of, 234. 

Rubies, 200. 

Ruby glass, 200. 


SALoL, 78. 

Salvarsan, 188. 

Salve-like, 91. 

Sauerbraten, 244. 

Secretion, 179. 

Selenium, 200. 

Semipermeable membranes, 48. 

Separation in critical fluids, 97; into 
phases, 99. 

Serum, 100. 

Sewage, 250. 

Silicic acid, 99; minerals of, 157; 
and Rontgen rays, 111. 

Silk, artificial, 226. 

Silver, 12; dispersion of, 34; hal- 
oids of, 64; colors of, 64, 68, 140; 
solutions of, 141; ‘colloids of, 188; 
in glass, 200. 

Silver chromate, 112, 113. 

Silver iodid, peptization of, 123. 

Sky, 152, 153. 

Sludge, 249. 

Smoke, 42, 249. 

Soap, 235. 

Sodium, swelling of, 101. 

Sodium chlorid, 6, 61. 

Sodium sulphate, 50. 

Soil chemistry, 161 to 165. 


SUBJECT INDEX 


Solid solution, 214. 

Solid + gas dispersoids, 42. 

Solid + liquid dispersoids, 40, 41, 86. 

Solid + solid dispersoids, 42. 

Sols, 86; mastic, 72. 

Solubility, 76. 

Solution, 11; of alkali metals, 141. 

Solvation, 52, 93, 142, 145, 146. 

Sorbite, 217. 

Specific surface, 77. 

Spontaneous ultrafilters, 48. 

Stabilization, 95, 98. 

Staining, 180. 

Starch, 101, 102, 235. 

Steel, 216 to 221. 

Straw, 197. 

Structure, 107, 108, 181. 

Sugar refining, 246. 

Sulphur, 12, 19, 41, 91, 201; color of, 
66; in rubber vulcanization, 229. 

Sulphur dyestuffs, 202. 

Superultrafiltration, 49. 

Surface, specific, 77; 
tion, 125, 126. 

Suspension colloids, 40, 86. 

Suspensions, 14, 40. 

Suspensoids, 40, 86. 

Swelling, 101; in vapor, 103; effect 
of added substances upon, 105; 
and syneresis, 106, 107, 108; 
cloudy, 176. 


and adsorp- 


Syneresis, 99, 106, 107; in bread, 243. 


Synthesis of rubber, 142, 230. 
Synthetic biology, 182. 


ae 


Tannic: Acip, 7. 

Tannin, 70, 197. 

Tanning, 221 to 225. 

Tantalum, 198. 

Temperature and colloids, 91. 

Tetraamyl-ammonium iodid, 7. 

Textiles, 221. 

Therapy, with inorganic colloids, 175; 
adsorption, 188. 

Time, 92. 


265 


Transition phenomena, 67. 

Transition systems, 54. 

Troostite, 219. 

Tungsten, 198. 

Turbidities, 50; of molecular solu- 
tions, 53. 

Tyndall cone, 51; and degree of dis- 
persion, 52; and molecularly dis- 
persed solutions, 54; of Roéntgen 
rays, 57. 

Types, of colloids, 40; of dispersed 
phases, 86, 111. 


U. 


ULTRAFILTERS, 48; SPONTANEOUS, 48. 
Ultrafiltration, 48, 57. 

Ultramarine, 201. 

Ultramicrons, 60. 

Ultramicroscope, 58, 59. 

Ultraviolet light Tyndall cone, 56. 
Ultraviolet rays, 53. 

Umber, 202. 

Uric acid, 188. 


ve 


VALENCE AND COAGULATION, 119. 
Variability in electrical behavior, 73. 
Vectorial forces, 46. 

Velocity, of colloid migration, 74; 
photophoretic, 154. 

Vital staining, 180. 

Violet gold, 23. 

Viscose gel, 99. 

Viscosity, 89; of emulsoids, 90, 91; 
of suspensoids, 90; of critical 
fluids, 96. 

Von Weimarn’s law, 26. 

Vulcanization, 229; theory of, 231; 
and ultraviolet light, 233; with- 
out sulphur, 233; accelerators, 233. 


W. 


Waste Propucts, 249. 


Water content of protoplasm, 169. 


266 SUBJECT INDEX 


Water-isobutyric acid, 97. World of Neglected Dimensions, 251. 
Water-phenol, 94, 96. Wound healing, 188. 

Wave-lengths and color, 65. 
Wenzel’s law, 79. Y. 
Wetting, 212. 


Wood, 193. YELLOW OcuHRE, 202. 











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